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regexregex-lookarounds

Unmatch complete words if a negative lookahead is satisfied

I need to match only those words which doesn't have special characters like @ and :. For example:

  1. [email protected] shouldn't match
  2. list should return a valid match
  3. show should also return a valid match

I tried it using a negative lookahead \w+(?![@:])

But it matches gi out of [email protected] but it shouldn't match that too.

Solution

  • You may add \w to the lookahead:

    \w+(?![\w@:])

    The equivalent is using a word boundary:

    \w+\b(?![@:])

    Besides, you may consider adding a left-hand boundary to avoid matching words inside non-word non-whitespace chunks of text:

    ^\w+(?![\w@:])

    Or

    (?<!\S)\w+(?![\w@:])

    The ^ will match the word at the start of the string and (?<!S) will match only if the word is preceded with whitespace or start of string.

    See the regex demo.

    Why not (?<!\S)\w+(?!\S), the whitespace boundaries? Because since you are building a lexer, you most probably have to deal with natural language sentences where words are likely to be followed with punctuation, and the (?!\S) negative lookahead would make the \w+ match only when it is followed with whitespace or at the end of the string.