Regular Expression

A regular expression is a formal language for describing string patterns, defining matching rules with metacharacters and quantifiers; it is widely used for search, validation, and extraction.

Purpose

Provides a compact yet powerful string pattern-matching language, replacing hand-written parsing logic.

Mechanism

Composed of literal characters and metacharacters (. * + ? [] () {} \ | ^ $). Quantifiers control repetition (* zero or more, + one or more, ? zero or one, {n,m} a specified range). Grouping () captures, | alternates, [] is a character class, and \d \w \s are predefined classes. Engines match via NFA/DFA backtracking or automata.

Use cases

Form input validation (emails, phone numbers)
Log pattern extraction
Find and replace (IDEs, sed)
Lexer generation

Examples

Input
```
/\d{3}-\d{4}/
```
Output
```
Matches 123-4567
```
3 digits-4 digits

Common errors

The greedy quantifier .* matches too much: Use the non-greedy .*? or a precise character class

Related terms

Glob is shell-style file-path pattern matching that uses wildcards such as * ** ? [] to match file names; it is simpler than regex.

Wildcards are simplified pattern-matching characters: * matches any sequence and ? matches a single character; commonly used for file-name and domain matching.

Fuzzy search is a search technique tolerant of spelling variations, matching approximate results based on edit distance, n-grams, or phonetic algorithms.

Related tools

Test regular expressions with match highlighting, capture groups, and replace preview. Free, private, and 100% client-side.

Find and replace text using regular expressions. Free, private, 100% client-side.

Regex Explain — pure client-side tool. Privacy-first, no server round-trip.

Regex Visualize

Render a regex as a Thompson NFA state diagram in Graphviz DOT format — ε-transitions, branches and quantifier loops visible; paste into any graphviz renderer.

Regex Validate Pattern

Regex Validate Pattern — pure client-side tool. Privacy-first, no server round-trip.

Regex Test Batch

Regex Test Batch — pure client-side tool. Privacy-first, no server round-trip.

Regex Escape String

Regex Escape String — pure client-side tool. Privacy-first, no server round-trip.

References