$` Inserts the portion of the string that precedes the matched substring.
$' Inserts the portion of the string that follows the matched substring.
$n Where n is a positive integer less than 100, inserts the nth parenthesized submatch string, provided the first argument was a RegExp object. Note that this is 1-indexed. If a group n is not present (e.g., if group is 3), it will be replaced as a literal (e.g., $3).
$<Name> Where Name is a capturing group name. If the group is not in the match, or not in the regular expression, or if a string was passed as the first argument to replace instead of a regular expression, this resolves to a literal (e.g., $<Name>). Only available in browser versions supporting named capturing groups.