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
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.