This document might help with understanding mIRC regex:
http://www.mircscripts.org/showdoc.php?type=tutorial&id=989

Special Character Definitions
Note: not all of these may apply to mIRC but the vast majority do.

\ Quote the next metacharacter
^ Match the beginning of the line
. Match any character (except newline)
$ Match the end of the line (or before newline at the end)
| Alternation
() Grouping
[] Character class

• Match 0 or more times
• Match 1 or more times
  ? Match 1 or 0 times
  \l lowercase next char (think vi)
  \u uppercase next char (think vi)
  \L lowercase till \E (think vi)
  \U uppercase till \E (think vi)
  \w Match a "word" character (alphanumeric plus "_")
  \W Match a non-word character
  \s Match a whitespace character
  \S Match a non-whitespace character
  \d Match a digit character
  \D Match a non-digit character
  \b Match a word boundary
  \B Match a non-(word boundary)

---unsure about these---
{n} Match exactly n times
{n,} Match at least n times
{n,m} Match at least n but not more than m times
\t tab (HT, TAB)
\n newline (LF, NL)
\r return (CR)
\f form feed (FF)
\a alarm (bell) (BEL)
\e escape (think troff) (ESC)
\033 octal char (think of a PDP-11)
\x1B hex char
\c[ control char
\E end case modification (think vi)
\Q quote (disable) pattern metacharacters till \E
\A Match only at beginning of string
\Z Match only at end of string, or before newline at the end
\z Match only at end of string
\G Match only where previous m//g left off (works only with /g)