For questions about using regular expression (regex or regexp) in Joomla development. Regex is a sequence of characters that define a search pattern for use in pattern matching with strings, like "find and replace" operations.

A regular expression (regex or regexp for short) is a special text string for describing a search pattern. You can think of regular expressions as wildcards on steroids. You are probably familiar with wildcard notations such as *.txt to find all text files in a file manager. The regex equivalent is .*\.txt$.

Regular expressions are so useful in computing that the various systems to specify regular expressions have evolved to provide both a basic and extended standard for the grammar and syntax; modern regular expressions heavily augment the standard.

Many programming languages provide regular expression capabilities, some built-in, and others via a standard library.

Resources:
- http://www.regular-expressions.info
- https://en.wikipedia.org/wiki/Regular_expression
- http://php.net/manual/en/book.pcre.php