Numbered Backreferences| www.regular-expressions.info
In a regular expression, parentheses can be used to group regex tokens together and for creating backreferences. Backreferences allow you to reuse part of the regex match in the regex, or in the replacement text.| www.regular-expressions.info
If you have written regular expressions before, you know that the regex syntax can be hard to keep track of. Certainly when writing a regex pattern that uses plenty of grouping and alternation.| www.regexbuddy.com