Filters are simply doing pattern matching using the squirrel library's regular expressions (bugs and all). So the squirrel documentation, particularly this:
http://www.squirrel-lang.org/doc/sqstdlib3.html#d0e2183 will be of some help too.
\ Quote the next metacharacter
^ Match the beginning of the string
. Match any character
$ Match the end of the string
| Alternation
(subexp) Grouping (creates a capture)
(?:subexp) No Capture Grouping (no capture)
[] Character class
GREEDY CLOSURES.
* Match 0 or more times
+ Match 1 or more times
? Match 1 or 0 times
{n} Match exactly n times
{n,} Match at least n times
{n,m} Match at least n but not more than m times
ESCAPE CHARACTERS.
\t tab (HT, TAB)
\n newline (LF, NL)
\r return (CR)
\f form feed (FF)
PREDEFINED CLASSES.
\l lowercase next char
\u uppercase next char
\a letters
\A non letters
\w alphanumeric [_0-9a-zA-Z]
\W non alphanumeric [^_0-9a-zA-Z]
\s space
\S non space
\d digits
\D non nondigits
\x exadecimal digits
\X non exadecimal digits
\c control charactrs
\C non control charactrs
\p punctation
\P non punctation
\b word boundary
\B non word boundary
