Regular Expressions

A regular expression (regex) is a concise notation for describing patterns in text. Regular expressions and finite state machines are mathematically equivalent — every regular expression can be converted to an FSM, and every FSM can be described by a regular expression. Together, they define the class of regular languages.

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Why Regular Expressions Matter

Regular expressions are used extensively in:

• Compilers — lexical analysis (tokenising source code)
• Text editors and IDEs — find-and-replace with patterns
• Data validation — checking email addresses, phone numbers, postcodes
• Web scraping and log analysis — extracting structured data from text
• Command-line tools — grep, sed, awk

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Regex Syntax

The A-Level specification requires you to understand the following operators: