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regexstringregex-negationnon-greedy

Which would be better non-greedy regex or negated character class?

I need to match @anything_here@ from a string @anything_here@dhhhd@shdjhjs@. So I'd used following regex.

^@.*?@

or

^@[^@]*@

Both way it's work but I would like to know which one would be a better solution. Regex with non-greedy repetition or regex with negated character class?

Solution

  • Negated character classes should usually be prefered over lazy matching, if possible.

    If the regex is successful, ^@[^@]*@ can match the content between @s in a single step, while ^@.*?@ needs to expand for each character between @s.

    When failing (for the case of no ending @) most regex engines will apply a little magic and internally treat [^@]* as [^@]*+, as there is a clear cut border between @ and non-@, thus it will match to the end of the string, recognize the missing @ and not backtrack, but instantly fail. .*? will expand character for character as usual.

    When used in larger contexts, [^@]* will also never expand over the borders of the ending @ while this is very well possible for the lazy matching. E.g. ^@[^@]*a[^@]*@ won't match @bbbb@a@ while ^@.*?a.*?@ will.

    Note that [^@] will also match newlines, while . doesn't (in most regex engines and unless used in singleline mode). You can avoid this by adding the newline character to the negation - if it is not wanted.