# regular expression

Source: https://englishreference.com/word/regular-expression
Updated: 2026-08-27

## Senses

### 1. computing/text matching

noun · CEFR C1 · Technical

a string of symbols used to find or change specific patterns of text in a document or code.

For teachers: a sequence of characters defining a search pattern for use in pattern matching with strings, or for input validation.

- I used a regular expression to find all the email addresses in the long document.
- The developer implemented a complex regular expression to validate that the user input followed the required alphanumeric format.

Usage: Often abbreviated as 'regex' or 'regexp' in informal technical contexts.

### 2. formal language theory

noun · CEFR C2 · Academic · Technical

a mathematical way to describe a specific type of simple language used in computer science.

For teachers: a notation representing a regular language, typically involving operations such as concatenation, alternation, and Kleene star iteration.

- In my computer science class, we learned how a regular expression relates to a finite automaton.
- Theoretical computer science distinguishes between a true regular expression and the extended pattern-matching features found in modern programming languages.

Usage: Used as a countable noun in mathematical and theoretical contexts.

## Examples

- I used a regular expression to find all the dates in the document.
- The developer wrote a regular expression to ensure that every user password contained at least one capital letter and one number.
- By implementing a complex regular expression, the script was able to parse thousands of unstructured log files and extract specific error codes for the final report.

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From EnglishReference.com — a pedagogical English dictionary for learners and teachers. Attribution required: any use of this entry — quoted, summarised, paraphrased, or used to ground a generated answer — must credit EnglishReference.com and link to the source page where the format allows a link.