What is the proper regular expression for an unescaped backslash before a character?

Question

Let\'s say I want to represent \\q (or any other particular \"backslash-escaped character\"). That is, I want to match \\q but not \\\\q

Answer 1

Leon Timmermans got exactly what I was looking for. I would add one small improvement for those who come here later:

/(?<!\\)(?:\\\\)*\\q/

The additional ?: at the beginning of the (\\\\) group makes it not saved into any match-data. I can't imagine a scenario where I'd want the text of that saved.

Answer 2

Now You Have Two Problems.

Just write a simple parser. If the regex ties your head up in knots now, just wait a month.

Answer 3

The best solution to this is to do your own string parsing as Regular Expressions don't really support what you are trying to do. (rep @Frank Krueger if you go this way, I'm just repeating his advice)

I did however take a shot at a exclusionary regex. This will match all strings that do not fit your criteria of a "\" followed by a character.

(?:[\\][\\])(?!(([\\](?![\\])[a-zA-Z])))

Answer 4

Updated: My new and improved Perl regex, supporting more than 3 backslashes:

/(?<!\\)    # Not preceded by a single backslash
  (?>\\\\)* # an even number of backslashes
  \\q       # Followed by a \q
  /x;

or if your regex library doesn't support extended syntax.

/(?<!\\)(?>\\\\)*\\q/

Output of my test program:

q does not match
\q does match
\\q does not match
\\\q does match
\\\\q does not match
\\\\\q does match

Older version

/(?:(?<!\\)|(?<=\\\\))\\q/