How to ignore comments inside string literals

Perhaps this might be another option.

Match 0+ times any character except a backslash, dot or exclamation mark using the first negated character class.

Then when you do match a character that the first character class does not matches, use an alternation to match either:

• repeat 0+ times matching either a dot that is not directly followed by 2 dots
• or match from 3 dots to the next first match of 3 dots
• or match only an escaped character

To prevent catastrophic backtracking, you can mimic an atomic group in Python using a positive lookahead with a capturing group inside. If the assertion is true, then use the backreference to \1 to match.

For example

(?<!\\)![^!\\.]*(?:(?:\.(?!\.\.)|(?=(\.{3}.*?\.{3}))\1|\\.)[^!\\.]*)*!

Explanation

• (?<!\\)! Match ! not directly preceded by \
• [^!\\.]* Match 1+ times any char except ! \ or .
• (?: Non capture group
  • (?:\.(?!\.\.) Match a dot not directly followed by 2 dots
  • | Or
  • (?=(\.{3}.*?\.{3}))\1 Assert and capture in group 1 from ... to the nearest ...
  • | Or
  • \\. Match an escaped char
• ) Close group
• [^!\\.]* Match 1+ times any char except ! \ or .
• )*! Close non capture group and repeat 0+ times, then match !

Regex demo