Improve RegEx documentation

[plokmijnuhby]

The current documentation is okay as far as it goes, but in terms of how to write a regex, it simply states, "For a more in-depth look, you can easily find various tutorials and detailed explanations on the Internet." This is not good enough, because while most implementations of regex support *, ?, and other common patterns, the exact details vary between systems. Are (?R) groups supported? What metacharacters are available? Are matches multiline, are they case sensitive? Does . match newlines? Can we have at least some information on what Godot regexes actually are?

[Calinou]

The regex module in Godot is based on PCRE2, but I don't know if there are specific caveats with regexes in Godot.

[plokmijnuhby]

Could you add a note to that effect? Perhaps you could point people to this page, or this one.

[bojidar-bg]

I guess we could also link people to https://www.pcre.org/current/doc/html/pcre2pattern.html.

[rslepon]

Version 2.1 documentation (https://docs.godotengine.org/en/2.1/classes/class_regex.html) had a list of supported features:
"Currently supported features:

• Capturing () and non-capturing (?:) groups
• Any character .
• Shorthand character classes \w \W \s \S \d \D
• User-defined character classes such as [A-Za-z]
• Simple quantifiers ?, \* and +
• Range quantifiers {x,y}
• Lazy (non-greedy) quantifiers \*?
• Beginning ^ and end $ anchors
• Alternation |
• Backreferences \1 and \g{1}
• POSIX character classes [[:alnum:]]
• Lookahead (?=), (?!) and lookbehind (?<=), (?<!)
• ASCII \xFF and Unicode \uFFFF code points (in a style similar to Python)
• Word boundaries \b, \B"

Perhaps it could be copied into the 3.x documentation and modified, if necessary. If doing so, note that \* in bullet 5 should be * and \*? in bullet 7 should be *?, I believe.