What does flag g mean in regex?

The global flag g instructs the regex engine to search for all matching occurrences in the string rather than stopping after the first match.

What does flag i mean in regex?

The i flag enables case-insensitive matching so that uppercase and lowercase letters are treated identically.

How to write regex for email validation?

A standard regex for email validation is ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$

How to match digits only in regex?

Use \d+ or [0-9]+ to match one or more numerical digits.

What is a lookahead in regex?

A lookahead (?=...) asserts that a specific pattern follows the current position without consuming characters in the match.

How to test regex online for JavaScript?

Type your pattern and test string into our Regex Tester tool for real-time match highlighting and group extraction.

Regex Tester & Debugger - Test Regular Expressions

01

Understanding Regular Expression Syntax & Mechanics

A Regular Expression (Regex) is a sequence of characters defining a search pattern. Modern JavaScript RegExp engines parse patterns into tokens, anchors, and quantifiers. Format JSON output with our <a href="/json-formatter" class="text-blue-400 underline hover:text-blue-300">JSON Formatter</a> when analyzing API responses.

02

Character Classes & Shorthands

Character classes match specific sets of characters. Standard shorthands include \d (digits 0-9), \w (alphanumeric characters and underscores), and \s (whitespace). Inverting shorthands to uppercase (\D, \W, \S) matches everything outside that set.

03

Greedy vs. Lazy Quantifiers

Quantifiers like *, +, and {n,m} specify repetition. Greedy quantifiers consume as much text as possible. Adding a question mark (*? or +?) makes quantifiers lazy, matching the minimum characters required.

04

Capturing Groups & Named Groups

Parentheses ( ) capture sub-matches into numbered variables ($1, $2). Named capture groups (?<name>...) assign clear object keys to extracted fields. Inspect JWT tokens and claim structures with our <a href="/jwt-decoder" class="text-blue-400 underline hover:text-blue-300">JWT Decoder</a>.

05

Lookahead & Lookbehind Assertions

Lookarounds match positions based on what follows or precedes them without consuming characters: (?=...) positive lookahead, (?!...) negative lookahead, (?<=...) positive lookbehind, and (?<!...) negative lookbehind. Encode/decode Base64 strings with our <a href="/base64-encoder-decoder" class="text-blue-400 underline hover:text-blue-300">Base64 Encoder/Decoder</a>.

06

Preventing Catastrophic Backtracking (ReDoS)

Catastrophic backtracking occurs when ambiguous nested quantifiers (such as (a+)+) force the engine into exponential state evaluations. Our tester includes automatic performance checks to alert you before UI execution freezes. Format SQL queries with our <a href="/sql-formatter" class="text-blue-400 underline hover:text-blue-300">SQL Formatter</a> or check <a href="/tools" class="text-blue-400 underline hover:text-blue-300">HTTP Status Codes</a>.

Regex Tester & Debugger

Regex Tester & Debugger

Expression Configuration

Regex Syntax Error

Performance Warning

Match Results & Structural Debugger

Comprehensive Guide to Regular Expressions & Debugging