What is a regular expression?

A regular expression (regex or regexp) is a sequence of characters that defines a search pattern. Regex engines use these patterns to find, match, or manipulate text. For example, the pattern /\d{3}-\d{4}/ matches a 7-digit phone number like '555-1234'. Regular expressions are built into almost every programming language and are used for input validation, search-and-replace, log parsing, and data extraction.

What are the most useful regex patterns?

Common patterns: Email: /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/ — URL: /https?:\/\/(www\.)?[\w-]+\.[a-z]{2,}([\w\-._~:/?#\[\]@!$&'()*+,;=]*)/ — IPv4 address: /^(\d{1,3}\.){3}\d{1,3}$/ — US phone number: /^\+?1?[-.\s]?$?\d{3}$?[-.\s]?\d{3}[-.\s]?\d{4}$/ — Only alphanumeric: /^[a-zA-Z0-9]+$/

What is the difference between greedy and lazy quantifiers?

Greedy quantifiers (*, +, ?) match as much text as possible. Lazy quantifiers (*?, +?, ??) match as little as possible. For example, given the string ' bold and italic ', the greedy pattern / / matches the entire string from the first . The lazy pattern / / matches ' ' and ' ' as separate matches. Use lazy quantifiers when you need to match the shortest possible string between delimiters.

What are regex flags and what do they do?

Flags modify how the regex engine interprets the pattern. The most common: g (global) — find all matches instead of stopping at the first. i (case insensitive) — treat A and a as the same character. m (multiline) — ^ and $ match line starts/ends instead of string start/end. s (dotAll) — make . match newline characters too. The tester above supports all standard JavaScript regex flags.

Text & String Tools

Regex Tester

Test regular expressions with live highlighting and match groups — runs entirely in your browser.

Pattern

//g

gglobaliinsensitivemmultilinesdotAll

Test String

Regular expressions — frequently asked questions

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