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, since the latter is a backslash-escaped backslash followed by a q. Yet \\\q would match, since it's a backslash-escaped backslash followed by a backslash-escaped q. (Well, it would match the \q at the end, not the \\ at the beginning.)

I know I need a negative lookbehind, but they always tie my head up in knots, especially since the backslashes themselves have to be escaped in the regexp.

Answer 1

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 2

To clarify for all the others that seem to have trouble understanding the question:

He's looking for a single character (non-space?) preceded by an ODD number of '\' characters.

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 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/

Answer 5

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.