what does this mean? what string will be matched using the expression?

I checked the sizzle code and see a definition.

var chunker = /((?:\((?:\([^()]+\)|[^()]+)+\)|\[(?:\[[^\[\]]*\]|['"][^'"]*['"]|[^\[\]'"]+)+\]|\\.|[^ >+~,(\[\\]+)+|[>+~])(\s*,\s*)?((?:.|\r|\n)*)/g,

I want to know how to find out what string(s) this regular expression will match?

Solution

See this article. Explanation in a multiline regex:

var chunker = /
 (
  (?:
   # One or more sets of parentheses that contain a string, or another set of 
   parentheses with a string
   \(
   (?:
    \([^()]+\)
    |
    [^()]+
   )+
   \)
   |
   # Or one or more sets of brackets that contain a string, or another set of
   brackets with a string
   \[
   (?:
    \[[^\[\]]*\]
    |
    ['"][^'"]*['"]
    |
    [^\[\]'"]+
   )+
   \]
   |
   # Or a backslash followed by any character
   \\.
   |
   # Or one or more of any except these characters: > +~,([\
   [^ >+~,(\[\\]+
  )+
  # or any one of these characters: >+~
  |
  [>+~]
 )
 # followed by zero or one commas, which may be surrounded by whitespace
 (\s*,\s*)?
 # followed by zero or more of anything, including line endings
 ((?:.|\r|\n)*)
/g

This expression contains three matching groups: A "validated" selector expression, eventual comma, and everything after that. It will continuosly be called on the selector to split it up in parts, see the Sizzle constructor for details.