Some regex best practices

With the regex functions, there are some best practices we could advise on /often made "mistakes" which we could warn people about.

I'm thinking along the lines of:

• Use of / as delimiter -> advise to use other delimiter
• Use of variables in the regex -> use preg_quote()
• Use of g modifier -> Not used in PHP use preg_match_all() function instead
• Use of s and m modifier in one regex -> they don't go together, review regex

Additionally the sniff could potentially check for:

• non-PCRE syntaxes used in the regex
• modifiers which are not implemented/available in PHP

This sniff would go into the extra ruleset.

Opinions ?

Start by reviewing the extra ruleset, which the issue identifies as the intended home for the sniff. The scope is not decided: clarify which regex practices and syntax or modifier checks are required, then define how completion will be verified; no file or test is named.