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

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

/(?\\\\)* # 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

/(?:(?