I need to match only those words which doesn't have special characters like @ and :.
For example:
git@github.com shouldn't matchlist should return a valid matchshow should also return a valid matchI tried it using a negative lookahead \w+(?![@:])
But it matches gi out of git@github.com but it shouldn't match that too.
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.
You can use negative lookbehind and negative lookahead patterns around a word pattern to make sure that the word is not preceded or followed by a non-space character, or in other words, to make sure that it is surrounded by either a space or a string boundary:
(?<!\S)\w+(?!\S)