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While working on a bash script today, I ran into two tasks for which (GNU) grep provided an elegant solution.
The first task was to check if a nonempty string contained only digits, not all zero, with leading zeroes allowed. If the check passed, the string remained the same; otherwise the string was emptied. This check could be done in regex by using negative lookahead:
^(?!0+$)\d+$
However, I couldn't use this regex directly with bash builtins such as double brackets or the expr command. The shell treated the exclamation point in the lookahead as a history expansion before evaluating the regex, which of course threw an error. My solution involved echoing the string and piping it into grep:
string="$(echo "$string" | grep -P '^(?!0+$)\d+