Regex Tester

The Complete Guide to Regular Expressions (Regex)

Regular expressions — regex or regexp for short — are one of the most powerful and most feared tools in a programmer's arsenal. A single regex can replace dozens of lines of string-parsing code, validate complex input formats, extract structured data from unstructured text, and perform find-and-replace operations that would otherwise require loops and conditionals. Our free online Regex Tester lets you write, test, and debug regular expressions in real time using the JavaScript ECMAScript regex engine.

Regex Syntax: The Core Building Blocks

Literal Characters

The simplest regex is just a literal string. The pattern hello matches the exact sequence of characters "hello" anywhere in the input text. Most characters match themselves literally.

Special Characters (Metacharacters)

These characters have special meaning in regex and must be escaped with a backslash \ to match them literally:

. ^ $ * + ? { } [ ] \ | ( )

For example, to match a literal period, use \. — because unescaped . matches any single character.

The Dot: Match Any Character

. matches any single character except a newline. c.t matches "cat", "cut", "cot", "c3t", "c t", but NOT "ct" (nothing between c and t).

Character Classes [ ]

Square brackets define a set of characters, any one of which can match at that position:

• [aeiou] — matches any single vowel
• [0-9] — matches any single digit (range syntax)
• [a-zA-Z] — matches any uppercase or lowercase letter
• [^aeiou] — the ^ inside [] negates — matches any character that is NOT a vowel

Shorthand Character Classes

• \d — digit: equivalent to [0-9]
• \D — non-digit: equivalent to [^0-9]
• \w — word character: equivalent to [a-zA-Z0-9_]
• \W — non-word character
• \s — whitespace: space, tab, newline, carriage return
• \S — non-whitespace

Anchors: Matching Position, Not Characters

• ^ — matches the start of a string (or start of a line with m flag)
• $ — matches the end of a string (or end of a line with m flag)
• \b — word boundary: matches the position between a word character and a non-word character
• \B — non-word boundary

Quantifiers: How Many Times?

• * — zero or more times
• + — one or more times
• ? — zero or one time (makes it optional)
• {3} — exactly 3 times
• {2,5} — between 2 and 5 times
• {3,} — 3 or more times

Greedy vs Lazy: By default, quantifiers are greedy — they match as much as possible. Adding ? after a quantifier makes it lazy — it matches as little as possible. Example: given <b>bold</b>, the pattern <.*> greedily matches the entire string, while <.*?> lazily matches just <b>.

Groups and Alternation

• (abc) — capturing group: groups the pattern and captures the match for backreference
• (?:abc) — non-capturing group: groups without capturing (more efficient)
• a|b — alternation: matches either "a" or "b"
• \1 — backreference: matches the same text that group 1 captured

Regex Flags (Modifiers)

• g (global) — find all matches, not just the first
• i (case-insensitive) — Hello matches "hello", "HELLO", "HeLLo"
• m (multiline) — ^ and $ match start/end of each line, not just the whole string
• s (dotAll) — makes . match newline characters too
• u (unicode) — treat pattern as Unicode code points

Essential Real-World Regex Patterns

• Email: ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$
• US Phone: ^\(?\d{3}\)?[-.\s]?\d{3}[-.\s]?\d{4}$
• URL: https?:\/\/(www\.)?[-a-zA-Z0-9@:%._+~#=]{2,256}\.[a-z]{2,6}\b([-a-zA-Z0-9@:%_+.~#?&//=]*)
• Date (YYYY-MM-DD): ^\d{4}-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01])$
• IPv4 Address: ^(\d{1,3}\.){3}\d{1,3}$
• Hex Color: ^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$
• Strong Password: ^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,}$

Use our free Regex Tester to write, test, and debug regular expressions in real time. For string manipulation tasks, also try our Text Diff Checker and Case Converter.