A regular expression (regex or regexp) is a sequence of characters that defines a search pattern, used for matching, extracting, and manipulating text in strings. Regexes are supported in virtually every programming language, text editor, and command-line tool.
Basic syntax
A regex pattern is built from literal characters and metacharacters:
.matches any single character*means zero or more of the preceding element+means one or more?means zero or one^anchors to the start of a string$anchors to the end[abc]matches any one of a, b, or c\dmatches any digit,\wmatches word characters,\smatches whitespace
Pattern: ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$
Matches: user@example.comCapture groups and backreferences
Parentheses () create capture groups that extract matched substrings. In the pattern (\d{4})-(\d{2})-(\d{2}) matched against 2026-03-21, group 1 captures 2026, group 2 captures 03, and group 3 captures 21.
Named groups like (?<year>\d{4}) make patterns more readable. Backreferences like \1 refer back to previously captured groups within the same pattern.
Lookaheads and lookbehinds
These are zero-width assertions — they check what’s around a match without including it:
(?=...)positive lookahead: match only if followed by…(?!...)negative lookahead: match only if NOT followed by…(?<=...)positive lookbehind: match only if preceded by…
Example: \d+(?=px) matches 16 in 16px but not 16 in 16em.
Where regex is used
- Validation: Email addresses, phone numbers, URLs, dates
- Search and replace: In code editors,
sed, and string manipulation functions - Log parsing: Extracting timestamps, error codes, IP addresses from log files
- Web scraping: Pulling structured data from HTML (though a proper parser is usually better)
- Routing: Web frameworks use regex patterns for URL routing
Common gotchas
Greedy matching (.*) captures as much as possible. Use .*? for non-greedy (lazy) matching. Backtracking can cause catastrophic performance on certain patterns — a concept called ReDoS (Regular Expression Denial of Service).
Test and debug patterns with the Regex Tester, reference syntax with the Regex Cheatsheet, or visualize pattern logic with the Regex Visualizer.