💾 Archived View for blitter.com › apl-books › APLX50 › APLX-manual › www.microapl.com › apl_help › p… captured on 2023-01-29 at 14:38:33.
⬅️ Previous capture (2022-07-17)
-=-=-=-=-=-=-
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<TITLE>Regular Expression Syntax</TITLE>
<META NAME="DESCRIPTION" CONTENT="APL language help page: Regular Expression Syntax">
<META NAME="KEYWORDS" CONTENT="string,search,replace,character,regular expression,regex,PCRE,apl,aplx,apl help">
<!-- %%COMMON_HEAD%% -->
<META http-equiv="Content-Type" content="text/html; charset=utf-8">
<LINK rel="stylesheet" type="text/css" href="http://www.microapl.com/styles_apl_help.css">
<!-- %%END%%-->
</HEAD>
<body>
<table>
<tr>
<td width="800" valign="top" colspan="2">
<center>Topic: <A HREF="ch.htm">APLX Help</A> : <A HREF="ch_205.htm">Regular Expressions in APLX</A> : <A HREF="pcrepattern_aplx.htm">Regular Expression Syntax</A>
</center>
<center>
[ <A HREF="ch_205_010.htm">Previous</A> | <A HREF="ch.htm">Contents</A> | <A HREF="help_index.htm">Index</A> | <A HREF="http://www.microapl.co.uk/apl/index.html">APL Home</A> ]</center>
<br></td>
</tr>
<tr>
<td width="120">
<a href="http://www.microapl.co.uk/apl/index.html"><img height="68" border="0" width="119" src="MicroAPL_logo.gif" alt="www.microapl.co.uk"></a>
</td>
<td align="left" valign="bottom">
<h1>Regular Expression Syntax</h1>
</td>
</tr>
<tr>
<td width="800" valign="top" colspan="2">
<hr>
<br>
<p>This section, which is an excerpt from the full PCRE documentation (Copyright © Philip Hazel), describes the syntax of regular-expression searches in PCRE, and thus in APLX. Some features which are not relevant to the use of PCRE in APLX have been edited out for clarity. </p>
<HR>
<ul>
<li><a name="TOC1" href="pcrepattern_aplx.htm#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>
<li><a name="TOC2" href="pcrepattern_aplx.htm#SEC2">CHARACTERS AND METACHARACTERS</a>
<li><a name="TOC3" href="pcrepattern_aplx.htm#SEC3">BACKSLASH</a>
<li><a name="TOC4" href="pcrepattern_aplx.htm#SEC4">CIRCUMFLEX AND DOLLAR</a>
<li><a name="TOC5" href="pcrepattern_aplx.htm#SEC5">FULL STOP (PERIOD, DOT)</a>
<li><a name="TOC6" href="pcrepattern_aplx.htm#SEC6">MATCHING A SINGLE BYTE</a>
<li><a name="TOC7" href="pcrepattern_aplx.htm#SEC7">SQUARE BRACKETS AND CHARACTER CLASSES</a>
<li><a name="TOC8" href="pcrepattern_aplx.htm#SEC8">POSIX CHARACTER CLASSES</a>
<li><a name="TOC9" href="pcrepattern_aplx.htm#SEC9">VERTICAL BAR</a>
<li><a name="TOC10" href="pcrepattern_aplx.htm#SEC10">INTERNAL OPTION SETTING</a>
<li><a name="TOC11" href="pcrepattern_aplx.htm#SEC11">SUBPATTERNS</a>
<li><a name="TOC12" href="pcrepattern_aplx.htm#SEC12">NAMED SUBPATTERNS</a>
<li><a name="TOC13" href="pcrepattern_aplx.htm#SEC13">REPETITION</a>
<li><a name="TOC14" href="pcrepattern_aplx.htm#SEC14">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a>
<li><a name="TOC15" href="pcrepattern_aplx.htm#SEC15">BACK REFERENCES</a>
<li><a name="TOC16" href="pcrepattern_aplx.htm#SEC16">ASSERTIONS</a>
<li><a name="TOC17" href="pcrepattern_aplx.htm#SEC17">CONDITIONAL SUBPATTERNS</a>
<li><a name="TOC18" href="pcrepattern_aplx.htm#SEC18">COMMENTS</a>
<li><a name="TOC19" href="pcrepattern_aplx.htm#SEC19">RECURSIVE PATTERNS</a>
<li><a name="TOC20" href="pcrepattern_aplx.htm#SEC20">SUBPATTERNS AS SUBROUTINES</a>
<li><a name="TOC23" href="pcrepattern_aplx.htm#SEC23">AUTHOR</a>
<li><a name="TOC24" href="pcrepattern_aplx.htm#SEC24">REVISION</a>
</ul>
<br><a name="SEC1" href="pcrepattern_aplx.htm#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>
<P>
The syntax and semantics of the regular expressions supported by PCRE are
described below. Regular expressions are also described in the Perl
documentation and in a number of books, some of which have copious examples.
Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers
regular expressions in great detail. This description of PCRE's regular
expressions is intended as reference material.
</P>
<br><a name="SEC2" href="pcrepattern_aplx.htm#TOC1">CHARACTERS AND METACHARACTERS</a><br>
<P>
A regular expression is a pattern that is matched against a subject string from
left to right. Most characters stand for themselves in a pattern, and match the
corresponding characters in the subject. As a trivial example, the pattern
<pre>
The quick brown fox
</pre>
matches a portion of a subject string that is identical to itself. When
caseless matching is specified, letters are matched
independently of case.
</P>
<P>
The power of regular expressions comes from the ability to include alternatives
and repetitions in the pattern. These are encoded in the pattern by the use of
<i>metacharacters</i>, which do not stand for themselves but instead are
interpreted in some special way.
</P>
<P>
There are two different sets of metacharacters: those that are recognized
anywhere in the pattern except within square brackets, and those that are
recognized within square brackets. Outside square brackets, the metacharacters
are as follows:
<pre>
\ general escape character with several uses
^ assert start of string (or line, in multiline mode)
$ assert end of string (or line, in multiline mode)
. match any character except newline (by default)
[ start character class definition
| start of alternative branch
( start subpattern
) end subpattern
? extends the meaning of (
also 0 or 1 quantifier
also quantifier minimizer
* 0 or more quantifier
+ 1 or more quantifier
also "possessive quantifier"
{ start min/max quantifier
</pre>
Part of a pattern that is in square brackets is called a "character class". In
a character class the only metacharacters are:
<pre>
\ general escape character
^ negate the class, but only if the first character
- indicates character range
[ POSIX character class (only if followed by POSIX syntax)
] terminates the character class
</pre>
The following sections describe the use of each of the metacharacters.
</P>
<br><a name="SEC3" href="pcrepattern_aplx.htm#TOC1">BACKSLASH</a><br>
<P>
The backslash character has several uses. Firstly, if it is followed by a
non-alphanumeric character, it takes away any special meaning that character
may have. This use of backslash as an escape character applies both inside and
outside character classes.
</P>
<P>
For example, if you want to match a * character, you write \* in the pattern.
This escaping action applies whether or not the following character would
otherwise be interpreted as a metacharacter, so it is always safe to precede a
non-alphanumeric with backslash to specify that it stands for itself. In
particular, if you want to match a backslash, you write \\.
</P>
<P>
If you want to remove the special meaning from a sequence of characters, you
can do so by putting them between \Q and \E. This is different from Perl in
that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in
Perl, $ and @ cause variable interpolation. Note the following examples:
<pre>
Pattern PCRE matches Perl matches
\Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
\Qabc\$xyz\E abc\$xyz abc\$xyz
\Qabc\E\$\Qxyz\E abc$xyz abc$xyz
</pre>
The \Q...\E sequence is recognized both inside and outside character classes.
<a name="digitsafterbackslash"></a></P>
<br><b>
Non-printing characters
</b><br>
<P>
A second use of backslash provides a way of encoding non-printing characters
in patterns in a visible manner. There is no restriction on the appearance of
non-printing characters, apart from the binary zero that terminates a pattern,
but when a pattern is being prepared by text editing, it is usually easier to
use one of the following escape sequences than the binary character it
represents:
<pre>
\a alarm, that is, the BEL character (hex 07)
\cx "control-x", where x is any character
\e escape (hex 1B)
\f formfeed (hex 0C)
\n newline (hex 0A)
\r carriage return (hex 0D)
\t tab (hex 09)
\ddd character with octal code ddd, or backreference
\xhh character with hex code hh
\x{hhh..} character with hex code hhh..
</pre>
The precise effect of \cx is as follows: if x is a lower case letter, it
is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c; becomes hex
7B.
</P>
<P>
After \x, from zero to two hexadecimal digits are read (letters can be in
upper or lower case). Any number of hexadecimal digits may appear between \x{
and }, but the value of the character code must be less than 256 in non-UTF-8
mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value
is 7FFFFFFF). If characters other than hexadecimal digits appear between \x{
and }, or if there is no terminating }, this form of escape is not recognized.
Instead, the initial \x will be interpreted as a basic hexadecimal escape,
with no following digits, giving a character whose value is zero.
</P>
<P>
Characters whose value is less than 256 can be defined by either of the two
syntaxes for \x. There is no difference in the way they are handled. For
example, \xdc is exactly the same as \x{dc}.
</P>
<P>
After \0 up to two further octal digits are read. If there are fewer than two
digits, just those that are present are used. Thus the sequence \0\x\07
specifies two binary zeros followed by a BEL character (code value 7). Make
sure you supply two digits after the initial zero if the pattern character that
follows is itself an octal digit.
</P>
<P>
The handling of a backslash followed by a digit other than 0 is complicated.
Outside a character class, PCRE reads it and any following digits as a decimal
number. If the number is less than 10, or if there have been at least that many
previous capturing left parentheses in the expression, the entire sequence is
taken as a <i>back reference</i>. A description of how this works is given
<a href="pcrepattern_aplx.htm#backreferences">later,</a>
following the discussion of
<a href="pcrepattern_aplx.htm#subpattern">parenthesized subpatterns.</a>
</P>
<P>
Inside a character class, or if the decimal number is greater than 9 and there
have not been that many capturing subpatterns, PCRE re-reads up to three octal
digits following the backslash, and uses them to generate a data character. Any
subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
character specified in octal must be less than \400. In UTF-8 mode, values up
to \777 are permitted. For example:
<pre>
\040 is another way of writing a space
\40 is the same, provided there are fewer than 40 previous capturing subpatterns
\7 is always a back reference
\11 might be a back reference, or another way of writing a tab
\011 is always a tab
\0113 is a tab followed by the character "3"
\113 might be a back reference, otherwise the character with octal code 113
\377 might be a back reference, otherwise the byte consisting entirely of 1 bits
\81 is either a back reference, or a binary zero followed by the two characters "8" and "1"
</pre>
Note that octal values of 100 or greater must not be introduced by a leading
zero, because no more than three octal digits are ever read.
</P>
<P>
All the sequences that define a single character value can be used both inside
and outside character classes. In addition, inside a character class, the
sequence \b is interpreted as the backspace character (hex 08), and the
sequences \R and \X are interpreted as the characters "R" and "X",
respectively. Outside a character class, these sequences have different
meanings
<a href="pcrepattern_aplx.htm#uniextseq">(see below).</a>
</P>
<br><b>
Absolute and relative back references
</b><br>
<P>
The sequence \g followed by a positive or negative number, optionally enclosed
in braces, is an absolute or relative back reference. Back references are
discussed
<a href="pcrepattern_aplx.htm#backreferences">later,</a>
following the discussion of
<a href="pcrepattern_aplx.htm#subpattern">parenthesized subpatterns.</a>
</P>
<br><b>
Generic character types
</b><br>
<P>
Another use of backslash is for specifying generic character types. The
following are always recognized:
<pre>
\d any decimal digit
\D any character that is not a decimal digit
\s any whitespace character
\S any character that is not a whitespace character
\w any "word" character
\W any "non-word" character
</pre>
Each pair of escape sequences partitions the complete set of characters into
two disjoint sets. Any given character matches one, and only one, of each pair.
</P>
<P>
These character type sequences can appear both inside and outside character
classes. They each match one character of the appropriate type. If the current
matching point is at the end of the subject string, all of them fail, since
there is no character to match.
</P>
<P>
For compatibility with Perl, \s does not match the VT character (code 11).
This makes it different from the the POSIX "space" class. The \s characters
are HT (9), LF (10), FF (12), CR (13), and space (32). (If "use locale;" is
included in a Perl script, \s may match the VT character. In PCRE, it never
does.)
</P>
<P>
A "word" character is an underscore or any character that is a
letter or digit.
</P>
<br><b>
Newline sequences
</b><br>
<P>
Outside a character class, the escape sequence \R matches any newline
sequence. This is an extension to Perl. \R is equivalent to
the following:
<pre>
(?>\r\n|\n|\x0b|\f|\r)
</pre>
This is an example of an "atomic group", details of which are given
<a href="pcrepattern_aplx.htm#atomicgroup">below.</a>
This particular group matches either the two-character sequence CR followed by
LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D). The two-character sequence is treated as a single unit that
cannot be split.
</P>
<P>
Inside a character class, \R matches the letter "R".
<a name="uniextseq"></a></P>
<br><b>
Simple assertions
</b><br>
<P>
The final use of backslash is for certain simple assertions. An assertion
specifies a condition that has to be met at a particular point in a match,
without consuming any characters from the subject string. The use of
subpatterns for more complicated assertions is described
<a href="pcrepattern_aplx.htm#bigassertions">below.</a>
The backslashed assertions are:
<pre>
\b matches at a word boundary
\B matches when not at a word boundary
\A matches at the start of the subject
\Z matches at the end of the subject
also matches before a newline at the end of the subject
\z matches only at the end of the subject
</pre>
These assertions may not appear in character classes (but note that \b has a
different meaning, namely the backspace character, inside a character class).
</P>
<P>
A word boundary is a position in the subject string where the current character
and the previous character do not both match \w or \W (i.e. one matches
\w and the other matches \W), or the start or end of the string if the
first or last character matches \w, respectively.
</P>
<P>
The \A, \Z, and \z assertions differ from the traditional circumflex and
dollar (described in the next section) in that they only ever match at the very
start and end of the subject string, whatever options are set. Thus, they are
independent of multiline mode. The
difference between \Z and \z is that \Z matches before a newline at the end
of the string as well as at the very end, whereas \z matches only at the end.
</P>
<br><a name="SEC4" href="pcrepattern_aplx.htm#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
<P>
Outside a character class, in the default matching mode, the circumflex
character is an assertion that is true only if the current matching point is
at the start of the subject string. Inside a character class, circumflex has an entirely different
meaning
<a href="pcrepattern_aplx.htm#characterclass">(see below).</a>
</P>
<P>
Circumflex need not be the first character of the pattern if a number of
alternatives are involved, but it should be the first thing in each alternative
in which it appears if the pattern is ever to match that branch. If all
possible alternatives start with a circumflex, that is, if the pattern is
constrained to match only at the start of the subject, it is said to be an
"anchored" pattern. (There are also other constructs that can cause a pattern
to be anchored.)
</P>
<P>
A dollar character is an assertion that is true only if the current matching
point is at the end of the subject string, or immediately before a newline
at the end of the string (by default). Dollar need not be the last character of
the pattern if a number of alternatives are involved, but it should be the last
item in any branch in which it appears. Dollar has no special meaning in a
character class.
</P>
<P>
The meaning of dollar can be changed so that it matches only at the very end of
the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
does not affect the \Z assertion.
</P>
<P>
The meanings of the circumflex and dollar characters are changed if the
multi-line search option is set. When this is the case, a circumflex matches
immediately after internal newlines as well as at the start of the subject
string. It does not match after a newline that ends the string. A dollar
matches before any newlines in the string, as well as at the very end, when
multi-line search is set. When newline is specified as the two-character
sequence CRLF, isolated CR and LF characters do not indicate newlines.
</P>
<P>
For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
\n represents a newline) in multiline mode, but not otherwise. Consequently,
patterns that are anchored in single line mode because all branches start with
^ are not anchored in multiline mode.
</P>
<P>
Note that the sequences \A, \Z, and \z can be used to match the start and
end of the subject in both modes, and if all branches of a pattern start with
\A it is always anchored, whether or not multi-line search is set.
</P>
<br><a name="SEC5" href="pcrepattern_aplx.htm#TOC1">FULL STOP (PERIOD, DOT)</a><br>
<P>
Outside a character class, a dot in the pattern matches any one character in
the subject string except (by default) a character that signifies the end of a
line.
</P>
<P>
When a line ending is defined as a single character, dot never matches that
character; when the two-character sequence CRLF is used, dot does not match CR
if it is immediately followed by LF, but otherwise it matches all characters
(including isolated CRs and LFs).
</P>
<P>
The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
option is set, a dot matches any one character, without exception. If the
two-character sequence CRLF is present in the subject string, it takes two dots
to match it.
</P>
<P>
The handling of dot is entirely independent of the handling of circumflex and
dollar, the only relationship being that they both involve newlines. Dot has no
special meaning in a character class.
</P>
<br><a name="SEC6" href="pcrepattern_aplx.htm#TOC1">MATCHING A SINGLE BYTE</a><br>
<P>
Outside a character class, the escape sequence \C matches any one byte. Unlike a dot, it always matches any line-ending
characters.
</P>
<br><a name="SEC7" href="pcrepattern_aplx.htm#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
<P>
An opening square bracket introduces a character class, terminated by a closing
square bracket. A closing square bracket on its own is not special. If a
closing square bracket is required as a member of the class, it should be the
first data character in the class (after an initial circumflex, if present) or
escaped with a backslash.
</P>
<P>
A character class matches a single character in the subject. A matched character must be in the set
of characters defined by the class, unless the first character in the class
definition is a circumflex, in which case the subject character must not be in
the set defined by the class. If a circumflex is actually required as a member
of the class, ensure it is not the first character, or escape it with a
backslash.
</P>
<P>
For example, the character class [aeiou] matches any lower case vowel, while
[^aeiou] matches any character that is not a lower case vowel. Note that a
circumflex is just a convenient notation for specifying the characters that
are in the class by enumerating those that are not. A class that starts with a
circumflex is not an assertion: it still consumes a character from the subject
string, and therefore it fails if the current pointer is at the end of the
string.
</P>
<P>
When caseless matching is set, any letters in a class represent both their
upper case and lower case versions, so for example, a caseless [aeiou] matches
"A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
caseful version would.
</P>
<P>
Characters that might indicate line breaks are never treated in any special way
when matching character classes, whatever line-ending sequence is in use. A class
such as [^a] always matches one of these characters.
</P>
<P>
The minus (hyphen) character can be used to specify a range of characters in a
character class. For example, [d-m] matches any letter between d and m,
inclusive. If a minus character is required in a class, it must be escaped with
a backslash or appear in a position where it cannot be interpreted as
indicating a range, typically as the first or last character in the class.
</P>
<P>
It is not possible to have the literal character "]" as the end character of a
range. A pattern such as [W-]46] is interpreted as a class of two characters
("W" and "-") followed by a literal string "46]", so it would match "W46]" or
"-46]". However, if the "]" is escaped with a backslash it is interpreted as
the end of range, so [W-\]46] is interpreted as a class containing a range
followed by two other characters. The octal or hexadecimal representation of
"]" can also be used to end a range.
</P>
<P>
Ranges operate in the collating sequence of character values. They can also be
used for characters specified numerically, for example [\000-\037].
</P>
<P>
If a range that includes letters is used when caseless matching is set, it
matches the letters in either case. For example, [W-c] is equivalent to
[][\\^_`wxyzabc], matched caselessly, and [\xc8-\xcb] matches accented E
characters in both cases.
</P>
<P>
The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
in a character class, and add the characters that they match to the class. For
example, [\dABCDEF] matches any hexadecimal digit. A circumflex can
conveniently be used with the upper case character types to specify a more
restricted set of characters than the matching lower case type. For example,
the class [^\W_] matches any letter or digit, but not underscore.
</P>
<P>
The only metacharacters that are recognized in character classes are backslash,
hyphen (only where it can be interpreted as specifying a range), circumflex
(only at the start), opening square bracket (only when it can be interpreted as
introducing a POSIX class name - see the next section), and the terminating
closing square bracket. However, escaping other non-alphanumeric characters
does no harm.
</P>
<br><a name="SEC8" href="pcrepattern_aplx.htm#TOC1">POSIX CHARACTER CLASSES</a><br>
<P>
Perl supports the POSIX notation for character classes. This uses names
enclosed by [: and :] within the enclosing square brackets. PCRE also supports
this notation. For example,
<pre>
[01[:alpha:]%]
</pre>
matches "0", "1", any alphabetic character, or "%". The supported class names
are
<pre>
alnum letters and digits
alpha letters
ascii character codes 0 - 127
blank space or tab only
cntrl control characters
digit decimal digits (same as \d)
graph printing characters, excluding space
lower lower case letters
print printing characters, including space
punct printing characters, excluding letters and digits
space white space (not quite the same as \s)
upper upper case letters
word "word" characters (same as \w)
xdigit hexadecimal digits
</pre>
The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
space (32). Notice that this list includes the VT character (code 11). This
makes "space" different to \s, which does not include VT (for Perl
compatibility).
</P>
<P>
The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
5.8. Another Perl extension is negation, which is indicated by a ^ character
after the colon. For example,
<pre>
[12[:^digit:]]
</pre>
matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
supported, and an error is given if they are encountered.
</P>
<br><a name="SEC9" href="pcrepattern_aplx.htm#TOC1">VERTICAL BAR</a><br>
<P>
Vertical bar characters are used to separate alternative patterns. For example,
the pattern
<pre>
gilbert|sullivan
</pre>
matches either "gilbert" or "sullivan". Any number of alternatives may appear,
and an empty alternative is permitted (matching the empty string). The matching
process tries each alternative in turn, from left to right, and the first one
that succeeds is used. If the alternatives are within a subpattern
<a href="pcrepattern_aplx.htm#subpattern">(defined below),</a>
"succeeds" means matching the rest of the main pattern as well as the
alternative in the subpattern.
</P>
<br><a name="SEC10" href="pcrepattern_aplx.htm#TOC1">INTERNAL OPTION SETTING</a><br>
<P>
The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
PCRE_EXTENDED options can be changed from within the pattern by a sequence of
Perl option letters enclosed between "(?" and ")". The option letters are
<pre>
i for PCRE_CASELESS
m for PCRE_MULTILINE
s for PCRE_DOTALL
x for PCRE_EXTENDED
</pre>
For example, (?im) sets caseless, multiline matching. It is also possible to
unset these options by preceding the letter with a hyphen, and a combined
setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
permitted. If a letter appears both before and after the hyphen, the option is
unset.
</P>
<P>
When an option change occurs at top level (that is, not inside subpattern
parentheses), the change applies to the remainder of the pattern that follows.
If the change is placed right at the start of a pattern, PCRE extracts it into
the global options (and it will therefore show up in data extracted by the
<b>pcre_fullinfo()</b> function).
</P>
<P>
An option change within a subpattern (see below for a description of
subpatterns) affects only that part of the current pattern that follows it, so
<pre>
(a(?i)b)c
</pre>
matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
By this means, options can be made to have different settings in different
parts of the pattern. Any changes made in one alternative do carry on
into subsequent branches within the same subpattern. For example,
<pre>
(a(?i)b|c)
</pre>
matches "ab", "aB", "c", and "C", even though when matching "C" the first
branch is abandoned before the option setting. This is because the effects of
option settings happen at compile time. There would be some very weird
behaviour otherwise.
</P>
<P>
The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
changed in the same way as the Perl-compatible options by using the characters
J, U and X respectively.
<a name="subpattern"></a></P>
<br><a name="SEC11" href="pcrepattern_aplx.htm#TOC1">SUBPATTERNS</a><br>
<P>
Subpatterns are delimited by parentheses (round brackets), which can be nested.
Turning part of a pattern into a subpattern does two things:
<br>
<br>
1. It localizes a set of alternatives. For example, the pattern
<pre>
cat(aract|erpillar|)
</pre>
matches one of the words "cat", "cataract", or "caterpillar". Without the
parentheses, it would match "cataract", "erpillar" or an empty string.
<br>
<br>
2. It sets up the subpattern as a capturing subpattern. This means that, when
the whole pattern matches, that portion of the subject string that matched the
subpattern is available as \1, \2 etc. Opening parentheses are counted from left to right (starting
from 1) to obtain numbers for the capturing subpatterns.
</P>
<P>
For example, if the string "the red king" is matched against the pattern
<pre>
the ((red|white) (king|queen))
</pre>
the captured substrings are "red king", "red", and "king", and are numbered 1,
2, and 3, respectively.
</P>
<P>
The fact that plain parentheses fulfil two functions is not always helpful.
There are often times when a grouping subpattern is required without a
capturing requirement. If an opening parenthesis is followed by a question mark
and a colon, the subpattern does not do any capturing, and is not counted when
computing the number of any subsequent capturing subpatterns. For example, if
the string "the white queen" is matched against the pattern
<pre>
the ((?:red|white) (king|queen))
</pre>
the captured substrings are "white queen" and "queen", and are numbered 1 and
2. The maximum number of capturing subpatterns is 65535.
</P>
<P>
As a convenient shorthand, if any option settings are required at the start of
a non-capturing subpattern, the option letters may appear between the "?" and
the ":". Thus the two patterns
<pre>
(?i:saturday|sunday)
(?:(?i)saturday|sunday)
</pre>
match exactly the same set of strings. Because alternative branches are tried
from left to right, and options are not reset until the end of the subpattern
is reached, an option setting in one branch does affect subsequent branches, so
the above patterns match "SUNDAY" as well as "Saturday".
</P>
<br><a name="SEC12" href="pcrepattern_aplx.htm#TOC1">NAMED SUBPATTERNS</a><br>
<P>
Identifying capturing parentheses by number is simple, but it can be very hard
to keep track of the numbers in complicated regular expressions. Furthermore,
if an expression is modified, the numbers may change. To help with this
difficulty, PCRE supports the naming of subpatterns. This feature was not
added to Perl until release 5.10. Python had the feature earlier, and PCRE
introduced it at release 4.0, using the Python syntax. PCRE now supports both
the Perl and the Python syntax.
</P>
<P>
In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
(?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
parentheses from other parts of the pattern, such as
<a href="pcrepattern_aplx.htm#backreferences">backreferences,</a>
<a href="pcrepattern_aplx.htm#recursion">recursion,</a>
and
<a href="pcrepattern_aplx.htm#conditions">conditions,</a>
can be made by name as well as by number.
</P>
<P>
Names consist of up to 32 alphanumeric characters and underscores. Named
capturing parentheses are still allocated numbers as well as names, exactly as
if the names were not present.
</P>
<br><a name="SEC13" href="pcrepattern_aplx.htm#TOC1">REPETITION</a><br>
<P>
Repetition is specified by quantifiers, which can follow any of the following
items:
<pre>
a literal data character
the dot metacharacter
the \C escape sequence
the \R escape sequence
an escape such as \d that matches a single character
a character class
a back reference (see next section)
a parenthesized subpattern (unless it is an assertion)
</pre>
The general repetition quantifier specifies a minimum and maximum number of
permitted matches, by giving the two numbers in curly brackets (braces),
separated by a comma. The numbers must be less than 65536, and the first must
be less than or equal to the second. For example:
<pre>
z{2,4}
</pre>
matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
character. If the second number is omitted, but the comma is present, there is
no upper limit; if the second number and the comma are both omitted, the
quantifier specifies an exact number of required matches. Thus
<pre>
[aeiou]{3,}
</pre>
matches at least 3 successive vowels, but may match many more, while
<pre>
\d{8}
</pre>
matches exactly 8 digits. An opening curly bracket that appears in a position
where a quantifier is not allowed, or one that does not match the syntax of a
quantifier, is taken as a literal character. For example, {,6} is not a
quantifier, but a literal string of four characters.
</P>
<P>
The quantifier {0} is permitted, causing the expression to behave as if the
previous item and the quantifier were not present.
</P>
<P>
For convenience, the three most common quantifiers have single-character
abbreviations:
<pre>
* is equivalent to {0,}
+ is equivalent to {1,}
? is equivalent to {0,1}
</pre>
It is possible to construct infinite loops by following a subpattern that can
match no characters with a quantifier that has no upper limit, for example:
<pre>
(a?)*
</pre>
Earlier versions of Perl and PCRE used to give an error at compile time for
such patterns. However, because there are cases where this can be useful, such
patterns are now accepted, but if any repetition of the subpattern does in fact
match no characters, the loop is forcibly broken.
</P>
<P>
By default, the quantifiers are "greedy", that is, they match as much as
possible (up to the maximum number of permitted times), without causing the
rest of the pattern to fail. The classic example of where this gives problems
is in trying to match comments in C programs. These appear between /* and */
and within the comment, individual * and / characters may appear. An attempt to
match C comments by applying the pattern
<pre>
/\*.*\*/
</pre>
to the string
<pre>
/* first comment */ not comment /* second comment */
</pre>
fails, because it matches the entire string owing to the greediness of the .*
item.
</P>
<P>
However, if a quantifier is followed by a question mark, it ceases to be
greedy, and instead matches the minimum number of times possible, so the
pattern
<pre>
/\*.*?\*/
</pre>
does the right thing with the C comments. The meaning of the various
quantifiers is not otherwise changed, just the preferred number of matches.
Do not confuse this use of question mark with its use as a quantifier in its
own right. Because it has two uses, it can sometimes appear doubled, as in
<pre>
\d??\d
</pre>
which matches one digit by preference, but can match two if that is the only
way the rest of the pattern matches.
</P>
<P>
If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
the quantifiers are not greedy by default, but individual ones can be made
greedy by following them with a question mark. In other words, it inverts the
default behaviour.
</P>
<P>
When a parenthesized subpattern is quantified with a minimum repeat count that
is greater than 1 or with a limited maximum, more memory is required for the
compiled pattern, in proportion to the size of the minimum or maximum.
</P>
<P>
If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
implicitly anchored, because whatever follows will be tried against every
character position in the subject string, so there is no point in retrying the
overall match at any position after the first. PCRE normally treats such a
pattern as though it were preceded by \A.
</P>
<P>
In cases where it is known that the subject string contains no newlines, it is
worth setting PCRE_DOTALL in order to obtain this optimization, or
alternatively using ^ to indicate anchoring explicitly.
</P>
<P>
However, there is one situation where the optimization cannot be used. When .*
is inside capturing parentheses that are the subject of a backreference
elsewhere in the pattern, a match at the start may fail where a later one
succeeds. Consider, for example:
<pre>
(.*)abc\1
</pre>
If the subject is "xyz123abc123" the match point is the fourth character. For
this reason, such a pattern is not implicitly anchored.
</P>
<P>
When a capturing subpattern is repeated, the value captured is the substring
that matched the final iteration. For example, after
<pre>
(tweedle[dume]{3}\s*)+
</pre>
has matched "tweedledum tweedledee" the value of the captured substring is
"tweedledee". However, if there are nested capturing subpatterns, the
corresponding captured values may have been set in previous iterations. For
example, after
<pre>
/(a|(b))+/
</pre>
matches "aba" the value of the second captured substring is "b".
<a name="atomicgroup"></a></P>
<br><a name="SEC14" href="pcrepattern_aplx.htm#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
<P>
With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
repetition, failure of what follows normally causes the repeated item to be
re-evaluated to see if a different number of repeats allows the rest of the
pattern to match. Sometimes it is useful to prevent this, either to change the
nature of the match, or to cause it fail earlier than it otherwise might, when
the author of the pattern knows there is no point in carrying on.
</P>
<P>
Consider, for example, the pattern \d+foo when applied to the subject line
<pre>
123456bar
</pre>
After matching all 6 digits and then failing to match "foo", the normal
action of the matcher is to try again with only 5 digits matching the \d+
item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
(a term taken from Jeffrey Friedl's book) provides the means for specifying
that once a subpattern has matched, it is not to be re-evaluated in this way.
</P>
<P>
If we use atomic grouping for the previous example, the matcher gives up
immediately on failing to match "foo" the first time. The notation is a kind of
special parenthesis, starting with (?> as in this example:
<pre>
(?>\d+)foo
</pre>
This kind of parenthesis "locks up" the part of the pattern it contains once
it has matched, and a failure further into the pattern is prevented from
backtracking into it. Backtracking past it to previous items, however, works as
normal.
</P>
<P>
An alternative description is that a subpattern of this type matches the string
of characters that an identical standalone pattern would match, if anchored at
the current point in the subject string.
</P>
<P>
Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
the above example can be thought of as a maximizing repeat that must swallow
everything it can. So, while both \d+ and \d+? are prepared to adjust the
number of digits they match in order to make the rest of the pattern match,
(?>\d+) can only match an entire sequence of digits.
</P>
<P>
Atomic groups in general can of course contain arbitrarily complicated
subpatterns, and can be nested. However, when the subpattern for an atomic
group is just a single repeated item, as in the example above, a simpler
notation, called a "possessive quantifier" can be used. This consists of an
additional + character following a quantifier. Using this notation, the
previous example can be rewritten as
<pre>
\d++foo
</pre>
Possessive quantifiers are always greedy. They are a convenient notation for the simpler forms of
atomic group. However, there is no difference in the meaning of a possessive
quantifier and the equivalent atomic group, though there may be a performance
difference; possessive quantifiers should be slightly faster.
</P>
<P>
The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
Jeffrey Friedl originated the idea (and the name) in the first edition of his
book. Mike McCloskey liked it, so implemented it when he built Sun's Java
package, and PCRE copied it from there. It ultimately found its way into Perl
at release 5.10.
</P>
<P>
PCRE has an optimization that automatically "possessifies" certain simple
pattern constructs. For example, the sequence A+B is treated as A++B because
there is no point in backtracking into a sequence of A's when B must follow.
</P>
<P>
When a pattern contains an unlimited repeat inside a subpattern that can itself
be repeated an unlimited number of times, the use of an atomic group is the
only way to avoid some failing matches taking a very long time indeed. The
pattern
<pre>
(\D+|<\d+>)*[!?]
</pre>
matches an unlimited number of substrings that either consist of non-digits, or
digits enclosed in <>, followed by either ! or ?. When it matches, it runs
quickly. However, if it is applied to
<pre>
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
</pre>
it takes a long time before reporting failure. This is because the string can
be divided between the internal \D+ repeat and the external * repeat in a
large number of ways, and all have to be tried. (The example uses [!?] rather
than a single character at the end, because both PCRE and Perl have an
optimization that allows for fast failure when a single character is used. They
remember the last single character that is required for a match, and fail early
if it is not present in the string.) If the pattern is changed so that it uses
an atomic group, like this:
<pre>
((?>\D+)|<\d+>)*[!?]
</pre>
sequences of non-digits cannot be broken, and failure happens quickly.
<a name="backreferences"></a></P>
<br><a name="SEC15" href="pcrepattern_aplx.htm#TOC1">BACK REFERENCES</a><br>
<P>
Outside a character class, a backslash followed by a digit greater than 0 (and
possibly further digits) is a back reference to a capturing subpattern earlier
(that is, to its left) in the pattern, provided there have been that many
previous capturing left parentheses.
</P>
<P>
However, if the decimal number following the backslash is less than 10, it is
always taken as a back reference, and causes an error only if there are not
that many capturing left parentheses in the entire pattern. In other words, the
parentheses that are referenced need not be to the left of the reference for
numbers less than 10. A "forward back reference" of this type can make sense
when a repetition is involved and the subpattern to the right has participated
in an earlier iteration.
</P>
<P>
It is not possible to have a numerical "forward back reference" to a subpattern
whose number is 10 or more using this syntax because a sequence such as \50 is
interpreted as a character defined in octal. See the subsection entitled
"Non-printing characters"
<a href="pcrepattern_aplx.htm#digitsafterbackslash">above</a>
for further details of the handling of digits following a backslash. There is
no such problem when named parentheses are used. A back reference to any
subpattern is possible using named parentheses (see below).
</P>
<P>
Another way of avoiding the ambiguity inherent in the use of digits following a
backslash is to use the \g escape sequence, which is a feature introduced in
Perl 5.10. This escape must be followed by a positive or a negative number,
optionally enclosed in braces. These examples are all identical:
<pre>
(ring), \1
(ring), \g1
(ring), \g{1}
</pre>
A positive number specifies an absolute reference without the ambiguity that is
present in the older syntax. It is also useful when literal digits follow the
reference. A negative number is a relative reference. Consider this example:
<pre>
(abc(def)ghi)\g{-1}
</pre>
The sequence \g{-1} is a reference to the most recently started capturing
subpattern before \g, that is, is it equivalent to \2. Similarly, \g{-2}
would be equivalent to \1. The use of relative references can be helpful in
long patterns, and also in patterns that are created by joining together
fragments that contain references within themselves.
</P>
<P>
A back reference matches whatever actually matched the capturing subpattern in
the current subject string, rather than anything matching the subpattern
itself (see
<a href="pcrepattern_aplx.htm#subpatternsassubroutines">"Subpatterns as subroutines"</a>
below for a way of doing that). So the pattern
<pre>
(sens|respons)e and \1ibility
</pre>
matches "sense and sensibility" and "response and responsibility", but not
"sense and responsibility". If caseful matching is in force at the time of the
back reference, the case of letters is relevant. For example,
<pre>
((?i)rah)\s+\1
</pre>
matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
capturing subpattern is matched caselessly.
</P>
<P>
Back references to named subpatterns use the Perl syntax \k<name> or \k'name'
or the Python syntax (?P=name). We could rewrite the above example in either of
the following ways:
<pre>
(?<p1>(?i)rah)\s+\k<p1>
(?P<p1>(?i)rah)\s+(?P=p1)
</pre>
A subpattern that is referenced by name may appear in the pattern before or
after the reference.
</P>
<P>
There may be more than one back reference to the same subpattern. If a
subpattern has not actually been used in a particular match, any back
references to it always fail. For example, the pattern
<pre>
(a|(bc))\2
</pre>
always fails if it starts to match "a" rather than "bc". Because there may be
many capturing parentheses in a pattern, all digits following the backslash are
taken as part of a potential back reference number. If the pattern continues
with a digit character, some delimiter must be used to terminate the back
reference. If the PCRE_EXTENDED option is set, this can be whitespace.
Otherwise an empty comment (see
<a href="pcrepattern_aplx.htm#comments">"Comments"</a>
below) can be used.
</P>
<P>
A back reference that occurs inside the parentheses to which it refers fails
when the subpattern is first used, so, for example, (a\1) never matches.
However, such references can be useful inside repeated subpatterns. For
example, the pattern
<pre>
(a|b\1)+
</pre>
matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
the subpattern, the back reference matches the character string corresponding
to the previous iteration. In order for this to work, the pattern must be such
that the first iteration does not need to match the back reference. This can be
done using alternation, as in the example above, or by a quantifier with a
minimum of zero.
<a name="bigassertions"></a></P>
<br><a name="SEC16" href="pcrepattern_aplx.htm#TOC1">ASSERTIONS</a><br>
<P>
An assertion is a test on the characters following or preceding the current
matching point that does not actually consume any characters. The simple
assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
<a href="pcrepattern_aplx.htm#smallassertions">above.</a>
</P>
<P>
More complicated assertions are coded as subpatterns. There are two kinds:
those that look ahead of the current position in the subject string, and those
that look behind it. An assertion subpattern is matched in the normal way,
except that it does not cause the current matching position to be changed.
</P>
<P>
Assertion subpatterns are not capturing subpatterns, and may not be repeated,
because it makes no sense to assert the same thing several times. If any kind
of assertion contains capturing subpatterns within it, these are counted for
the purposes of numbering the capturing subpatterns in the whole pattern.
However, substring capturing is carried out only for positive assertions,
because it does not make sense for negative assertions.
</P>
<br><b>
Lookahead assertions
</b><br>
<P>
Lookahead assertions start with (?= for positive assertions and (?! for
negative assertions. For example,
<pre>
\w+(?=;)
</pre>
matches a word followed by a semicolon, but does not include the semicolon in
the match, and
<pre>
foo(?!bar)
</pre>
matches any occurrence of "foo" that is not followed by "bar". Note that the
apparently similar pattern
<pre>
(?!foo)bar
</pre>
does not find an occurrence of "bar" that is preceded by something other than
"foo"; it finds any occurrence of "bar" whatsoever, because the assertion
(?!foo) is always true when the next three characters are "bar". A
lookbehind assertion is needed to achieve the other effect.
</P>
<P>
If you want to force a matching failure at some point in a pattern, the most
convenient way to do it is with (?!) because an empty string always matches, so
an assertion that requires there not to be an empty string must always fail.
<a name="lookbehind"></a></P>
<br><b>
Lookbehind assertions
</b><br>
<P>
Lookbehind assertions start with (?<= for positive assertions and (?<! for
negative assertions. For example,
<pre>
(?<!foo)bar
</pre>
does find an occurrence of "bar" that is not preceded by "foo". The contents of
a lookbehind assertion are restricted such that all the strings it matches must
have a fixed length. However, if there are several top-level alternatives, they
do not all have to have the same fixed length. Thus
<pre>
(?<=bullock|donkey)
</pre>
is permitted, but
<pre>
(?<!dogs?|cats?)
</pre>
causes an error at compile time. Branches that match different length strings
are permitted only at the top level of a lookbehind assertion. This is an
extension compared with Perl (at least for 5.8), which requires all branches to
match the same length of string. An assertion such as
<pre>
(?<=ab(c|de))
</pre>
is not permitted, because its single top-level branch can match two different
lengths, but it is acceptable if rewritten to use two top-level branches:
<pre>
(?<=abc|abde)
</pre>
The implementation of lookbehind assertions is, for each alternative, to
temporarily move the current position back by the fixed length and then try to
match. If there are insufficient characters before the current position, the
assertion fails.
</P>
<P>
PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode)
to appear in lookbehind assertions, because it makes it impossible to calculate
the length of the lookbehind. The \X and \R escapes, which can match
different numbers of bytes, are also not permitted.
</P>
<P>
Possessive quantifiers can be used in conjunction with lookbehind assertions to
specify efficient matching at the end of the subject string. Consider a simple
pattern such as
<pre>
abcd$
</pre>
when applied to a long string that does not match. Because matching proceeds
from left to right, PCRE will look for each "a" in the subject and then see if
what follows matches the rest of the pattern. If the pattern is specified as
<pre>
^.*abcd$
</pre>
the initial .* matches the entire string at first, but when this fails (because
there is no following "a"), it backtracks to match all but the last character,
then all but the last two characters, and so on. Once again the search for "a"
covers the entire string, from right to left, so we are no better off. However,
if the pattern is written as
<pre>
^.*+(?<=abcd)
</pre>
there can be no backtracking for the .*+ item; it can match only the entire
string. The subsequent lookbehind assertion does a single test on the last four
characters. If it fails, the match fails immediately. For long strings, this
approach makes a significant difference to the processing time.
</P>
<br><b>
Using multiple assertions
</b><br>
<P>
Several assertions (of any sort) may occur in succession. For example,
<pre>
(?<=\d{3})(?<!999)foo
</pre>
matches "foo" preceded by three digits that are not "999". Notice that each of
the assertions is applied independently at the same point in the subject
string. First there is a check that the previous three characters are all
digits, and then there is a check that the same three characters are not "999".
This pattern does <i>not</i> match "foo" preceded by six characters, the first
of which are digits and the last three of which are not "999". For example, it
doesn't match "123abcfoo". A pattern to do that is
<pre>
(?<=\d{3}...)(?<!999)foo
</pre>
This time the first assertion looks at the preceding six characters, checking
that the first three are digits, and then the second assertion checks that the
preceding three characters are not "999".
</P>
<P>
Assertions can be nested in any combination. For example,
<pre>
(?<=(?<!foo)bar)baz
</pre>
matches an occurrence of "baz" that is preceded by "bar" which in turn is not
preceded by "foo", while
<pre>
(?<=\d{3}(?!999)...)foo
</pre>
is another pattern that matches "foo" preceded by three digits and any three
characters that are not "999".
<a name="conditions"></a></P>
<br><a name="SEC17" href="pcrepattern_aplx.htm#TOC1">CONDITIONAL SUBPATTERNS</a><br>
<P>
It is possible to cause the matching process to obey a subpattern
conditionally or to choose between two alternative subpatterns, depending on
the result of an assertion, or whether a previous capturing subpattern matched
or not. The two possible forms of conditional subpattern are
<pre>
(?(condition)yes-pattern)
(?(condition)yes-pattern|no-pattern)
</pre>
If the condition is satisfied, the yes-pattern is used; otherwise the
no-pattern (if present) is used. If there are more than two alternatives in the
subpattern, a compile-time error occurs.
</P>
<P>
There are four kinds of condition: references to subpatterns, references to
recursion, a pseudo-condition called DEFINE, and assertions.
</P>
<br><b>
Checking for a used subpattern by number
</b><br>
<P>
If the text between the parentheses consists of a sequence of digits, the
condition is true if the capturing subpattern of that number has previously
matched.
</P>
<P>
Consider the following pattern, which contains non-significant white space to
make it more readable (assume the PCRE_EXTENDED option) and to divide it into
three parts for ease of discussion:
<pre>
( \( )? [^()]+ (?(1) \) )
</pre>
The first part matches an optional opening parenthesis, and if that
character is present, sets it as the first captured substring. The second part
matches one or more characters that are not parentheses. The third part is a
conditional subpattern that tests whether the first set of parentheses matched
or not. If they did, that is, if subject started with an opening parenthesis,
the condition is true, and so the yes-pattern is executed and a closing
parenthesis is required. Otherwise, since no-pattern is not present, the
subpattern matches nothing. In other words, this pattern matches a sequence of
non-parentheses, optionally enclosed in parentheses.
</P>
<br><b>
Checking for a used subpattern by name
</b><br>
<P>
Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
subpattern by name. For compatibility with earlier versions of PCRE, which had
this facility before Perl, the syntax (?(name)...) is also recognized. However,
there is a possible ambiguity with this syntax, because subpattern names may
consist entirely of digits. PCRE looks first for a named subpattern; if it
cannot find one and the name consists entirely of digits, PCRE looks for a
subpattern of that number, which must be greater than zero. Using subpattern
names that consist entirely of digits is not recommended.
</P>
<P>
Rewriting the above example to use a named subpattern gives this:
<pre>
(?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
</PRE>
</P>
<br><b>
Checking for pattern recursion
</b><br>
<P>
If the condition is the string (R), and there is no subpattern with the name R,
the condition is true if a recursive call to the whole pattern or any
subpattern has been made. If digits or a name preceded by ampersand follow the
letter R, for example:
<pre>
(?(R3)...) or (?(R&name)...)
</pre>
the condition is true if the most recent recursion is into the subpattern whose
number or name is given. This condition does not check the entire recursion
stack.
</P>
<P>
At "top level", all these recursion test conditions are false. Recursive
patterns are described below.
</P>
<br><b>
Defining subpatterns for use by reference only
</b><br>
<P>
If the condition is the string (DEFINE), and there is no subpattern with the
name DEFINE, the condition is always false. In this case, there may be only one
alternative in the subpattern. It is always skipped if control reaches this
point in the pattern; the idea of DEFINE is that it can be used to define
"subroutines" that can be referenced from elsewhere. (The use of "subroutines"
is described below.) For example, a pattern to match an IPv4 address could be
written like this (ignore whitespace and line breaks):
<pre>
(?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
\b (?&byte) (\.(?&byte)){3} \b
</pre>
The first part of the pattern is a DEFINE group inside which a another group
named "byte" is defined. This matches an individual component of an IPv4
address (a number less than 256). When matching takes place, this part of the
pattern is skipped because DEFINE acts like a false condition.
</P>
<P>
The rest of the pattern uses references to the named group to match the four
dot-separated components of an IPv4 address, insisting on a word boundary at
each end.
</P>
<br><b>
Assertion conditions
</b><br>
<P>
If the condition is not in any of the above formats, it must be an assertion.
This may be a positive or negative lookahead or lookbehind assertion. Consider
this pattern, again containing non-significant white space, and with the two
alternatives on the second line:
<pre>
(?(?=[^a-z]*[a-z])
\d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
</pre>
The condition is a positive lookahead assertion that matches an optional
sequence of non-letters followed by a letter. In other words, it tests for the
presence of at least one letter in the subject. If a letter is found, the
subject is matched against the first alternative; otherwise it is matched
against the second. This pattern matches strings in one of the two forms
dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
<a name="comments"></a></P>
<br><a name="SEC18" href="pcrepattern_aplx.htm#TOC1">COMMENTS</a><br>
<P>
The sequence (?# marks the start of a comment that continues up to the next
closing parenthesis. Nested parentheses are not permitted. The characters
that make up a comment play no part in the pattern matching at all.
</P>
<P>
If the PCRE_EXTENDED option is set, an unescaped # character outside a
character class introduces a comment that continues to immediately after the
next newline in the pattern.
<a name="recursion"></a></P>
<br><a name="SEC19" href="pcrepattern_aplx.htm#TOC1">RECURSIVE PATTERNS</a><br>
<P>
Consider the problem of matching a string in parentheses, allowing for
unlimited nested parentheses. Without the use of recursion, the best that can
be done is to use a pattern that matches up to some fixed depth of nesting. It
is not possible to handle an arbitrary nesting depth.
</P>
<P>
For some time, Perl has provided a facility that allows regular expressions to
recurse (amongst other things). It does this by interpolating Perl code in the
expression at run time, and the code can refer to the expression itself. A Perl
pattern using code interpolation to solve the parentheses problem can be
created like this:
<pre>
$re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
</pre>
The (?p{...}) item interpolates Perl code at run time, and in this case refers
recursively to the pattern in which it appears.
</P>
<P>
Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
supports special syntax for recursion of the entire pattern, and also for
individual subpattern recursion. After its introduction in PCRE and Python,
this kind of recursion was introduced into Perl at release 5.10.
</P>
<P>
A special item that consists of (? followed by a number greater than zero and a
closing parenthesis is a recursive call of the subpattern of the given number,
provided that it occurs inside that subpattern. (If not, it is a "subroutine"
call, which is described in the next section.) The special item (?R) or (?0) is
a recursive call of the entire regular expression.
</P>
<P>
In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
treated as an atomic group. That is, once it has matched some of the subject
string, it is never re-entered, even if it contains untried alternatives and
there is a subsequent matching failure.
</P>
<P>
This PCRE pattern solves the nested parentheses problem (assume the
PCRE_EXTENDED option is set so that white space is ignored):
<pre>
\( ( (?>[^()]+) | (?R) )* \)
</pre>
First it matches an opening parenthesis. Then it matches any number of
substrings which can either be a sequence of non-parentheses, or a recursive
match of the pattern itself (that is, a correctly parenthesized substring).
Finally there is a closing parenthesis.
</P>
<P>
If this were part of a larger pattern, you would not want to recurse the entire
pattern, so instead you could use this:
<pre>
( \( ( (?>[^()]+) | (?1) )* \) )
</pre>
We have put the pattern into parentheses, and caused the recursion to refer to
them instead of the whole pattern. In a larger pattern, keeping track of
parenthesis numbers can be tricky. It may be more convenient to use named
parentheses instead. The Perl syntax for this is (?&name); PCRE's earlier
syntax (?P>name) is also supported. We could rewrite the above example as
follows:
<pre>
(?<pn> \( ( (?>[^()]+) | (?&pn) )* \) )
</pre>
If there is more than one subpattern with the same name, the earliest one is
used. This particular example pattern contains nested unlimited repeats, and so
the use of atomic grouping for matching strings of non-parentheses is important
when applying the pattern to strings that do not match. For example, when this
pattern is applied to
<pre>
(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
</pre>
it yields "no match" quickly. However, if atomic grouping is not used,
the match runs for a very long time indeed because there are so many different
ways the + and * repeats can carve up the subject, and all have to be tested
before failure can be reported.
</P>
<P>
At the end of a match, the values set for any capturing subpatterns are those
from the outermost level of the recursion at which the subpattern value is set. If the pattern above is matched against
<pre>
(ab(cd)ef)
</pre>
the value for the capturing parentheses is "ef", which is the last value taken
on at the top level. If additional parentheses are added, giving
<pre>
\( ( ( (?>[^()]+) | (?R) )* ) \)
^ ^
^ ^
</pre>
the string they capture is "ab(cd)ef", the contents of the top level
parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
has to obtain extra memory to store data during a recursion. If no
memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
</P>
<P>
Do not confuse the (?R) item with the condition (R), which tests for recursion.
Consider this pattern, which matches text in angle brackets, allowing for
arbitrary nesting. Only digits are allowed in nested brackets (that is, when
recursing), whereas any characters are permitted at the outer level.
<pre>
< (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
</pre>
In this pattern, (?(R) is the start of a conditional subpattern, with two
different alternatives for the recursive and non-recursive cases. The (?R) item
is the actual recursive call.
<a name="subpatternsassubroutines"></a></P>
<br><a name="SEC20" href="pcrepattern_aplx.htm#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
<P>
If the syntax for a recursive subpattern reference (either by number or by
name) is used outside the parentheses to which it refers, it operates like a
subroutine in a programming language. The "called" subpattern may be defined
before or after the reference. An earlier example pointed out that the pattern
<pre>
(sens|respons)e and \1ibility
</pre>
matches "sense and sensibility" and "response and responsibility", but not
"sense and responsibility". If instead the pattern
<pre>
(sens|respons)e and (?1)ibility
</pre>
is used, it does match "sense and responsibility" as well as the other two
strings. Another example is given in the discussion of DEFINE above.
</P>
<P>
Like recursive subpatterns, a "subroutine" call is always treated as an atomic
group. That is, once it has matched some of the subject string, it is never
re-entered, even if it contains untried alternatives and there is a subsequent
matching failure.
</P>
<P>
When a subpattern is used as a subroutine, processing options such as
case-independence are fixed when the subpattern is defined. They cannot be
changed for different calls. For example, consider this pattern:
<pre>
(abc)(?i:(?1))
</pre>
It matches "abcabc". It does not match "abcABC" because the change of
processing option does not affect the called subpattern.
</P>
<br><a name="SEC23" href="pcrepattern_aplx.htm#TOC1">AUTHOR</a><br>
<P>
Philip Hazel
<br>
University Computing Service
<br>
Cambridge CB2 3QH, England.
<br>
</P>
<br><a name="SEC24" href="pcrepattern_aplx.htm#TOC1">REVISION</a><br>
<P>
Last updated: 06 March 2007
<br>
Copyright © 1997-2007 University of Cambridge.
<br>
<hr>
<H2><a name="SEC25">Copyright and Licence Details</a></H2>
<p>The following legal notice applies to the PCRE code used in APLX:</p>
<pre>PCRE is a library of functions to support regular expressions whose syntax
and semantics are as close as possible to those of the Perl 5 language.
Written by Philip Hazel
Copyright (c) 1997-2007 University of Cambridge
-----------------------------------------------------------------------------
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are met:
* Redistributions of source code must retain the above copyright notice,
this list of conditions and the following disclaimer.
* Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
* Neither the name of the University of Cambridge nor the names of its
contributors may be used to endorse or promote products derived from
this software without specific prior written permission.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
POSSIBILITY OF SUCH DAMAGE.</pre>
<P><BR></P>
<hr>
</td>
</tr>
<tr>
<td width="800" valign="top" colspan="2">
<center>Topic: <A HREF="ch.htm">APLX Help</A> : <A HREF="ch_205.htm">Regular Expressions in APLX</A> : <A HREF="pcrepattern_aplx.htm">Regular Expression Syntax</A>
</center>
<center>
[ <A HREF="ch_205_010.htm">Previous</A> | <A HREF="ch.htm">Contents</A> | <A HREF="help_index.htm">Index</A> | <A HREF="http://www.microapl.co.uk/apl/index.html">APL Home</A> ]</center>
<br></td>
</tr>
</table>
<!-- %%COMMON_BODY_TAIL%% -->
<p class="copyright">Copyright © 1996-2010 MicroAPL Ltd</p>
<!-- %%END%% -->
</body>
</html>