What is Regex?

Regex (short for regular expression) is a sequence of characters that defines a search pattern. Regex is used to match, find, replace or validate strings — from simple word searches to complex text processing tasks. Every major programming language includes a built-in regex engine, making it one of the most universally applicable tools in a developer's toolkit.

History

Regular expressions originate from formal language theory. Mathematician Stephen Cole Kleene introduced them in the 1950s as a notation for describing regular languages. Ken Thompson implemented regex in the UNIX text editor ed and later in grep (1973), making them widely available to developers. Perl (1987) expanded the syntax significantly and popularized the PCRE (Perl-Compatible Regular Expressions) dialect that most modern languages follow today.

Basic Syntax

A regular expression is a pattern written between delimiters (in JavaScript: /pattern/flags). The pattern can contain literal characters, special metacharacters and quantifiers. The regex engine scans a string and reports every position where the pattern matches.

/* Email validation regex */
^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$

/* Match: alice@example.com */
/* No match: notanemail */

Core Concepts

• Literals — Match exact characters: cat matches the string "cat"
• Character classes — Match one character from a set: [aeiou] matches any vowel, [a-z] any lowercase letter, [^0-9] any non-digit
• Anchors — ^ matches the start of a string, $ matches the end
• Quantifiers — * (0 or more), + (1 or more), ? (0 or 1), {n} (exactly n), {n,m} (between n and m)
• Wildcard — . matches any single character except a newline
• Alternation — cat|dog matches either "cat" or "dog"
• Groups — (abc) capturing group, (?:abc) non-capturing group
• Escape — \ before a metacharacter treats it as a literal: \. matches a period
• Shorthand classes — \d (digit), \w (word character), \s (whitespace), and their negations \D, \W, \S

Common Patterns

• Email — ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$
• URL — https?://[^\s]+
• Date (YYYY-MM-DD) — \d{4}-\d{2}-\d{2}
• Phone number — \+?[\d\s\-\(\)]{7,15}
• IPv4 address — \b\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\b
• Hex color — #[0-9a-fA-F]{3,6}
• Postal code (US) — \d{5}(-\d{4})?

Flags

Flags modify how the regex engine interprets the pattern:

• g — global — find all matches, not just the first
• i — case-insensitive — /hello/i matches "Hello", "HELLO", "hello"
• m — multiline — ^ and $ match the start and end of each line
• s — dotAll — . also matches newline characters
• u — unicode — enables full Unicode matching

Use Cases

• Input validation — Ensure email addresses, phone numbers, passwords and postal codes meet required formats
• Search and replace — Text editors like VS Code use regex to find and replace across files
• Log parsing — Extract IP addresses, timestamps and error codes from server logs
• URL routing — Web frameworks match incoming request paths against regex patterns
• Data extraction — Scrape structured data (prices, dates, links) from HTML or plain text
• Syntax highlighting — Code editors use regex to identify and color keywords, strings and comments

Rules to Remember

• Metacharacters . * + ? ^ $ { } [ ] | ( ) \ must be escaped with \ to be treated as literals
• Greedy quantifiers (*, +) match as much as possible; add ? after them (*?, +?) for lazy matching
• The ^ anchor means "start of string" outside a character class but "negation" inside one: [^abc]
• Regex is not the right tool for parsing HTML or XML — use a proper parser instead
• Test your patterns on real data with edge cases before deploying