Patterns
Patterns in configuration parameters and expressions use the standard Java regular expression Pattern class. For more information on regular expressions, see Oracle’s tutorial on Regular Expressions.
Pattern templates
A regular expression pattern template expresses a transformation to be applied
for a matching regular expression pattern. It may contain references to
capturing groups within the match result. Each occurrence of $g (where
g is an integer value) is substituted by the indexed capturing
group in a match result. Capturing group zero "$0" denotes the entire pattern
match. A dollar sign or numeral literal immediately following a capture group
reference can be included as a literal in the template by preceding it with a
backslash ( \ ). Backslash itself must be also escaped in this manner.
More information
Java Pattern class