What is a regular expression (regex)?

A regular expression is a sequence of characters that forms a search pattern. It is used in programming to validate inputs, search text logs, extract data patterns, and perform bulk search-and-replace actions.

What is the difference between greedy and lazy quantifiers?

Greedy quantifiers (like * or +) match as much text as possible. Lazy quantifiers (like *? or +?) match the minimum amount of text possible before satisfying the pattern.

What is catastrophic backtracking?

Catastrophic backtracking occurs in NFA regex engines when a pattern contains overlapping nested quantifiers (like (a+)+). When processing non-matching inputs, the engine tests every permutation, resulting in exponential execution times that freeze CPUs.

What do the g, i, and m flags do in regex?

The "g" (global) flag searches for all matches in the text rather than stopping after the first match. The "i" flag makes the search case-insensitive. The "m" (multi-line) flag makes the anchors ^ and $ match the start and end of individual lines.

What is the difference between capturing and non-capturing groups?

Capturing groups "(...)" remember the matched text for later extraction or backreferencing. Non-capturing groups "(?:...)" group elements for quantifiers without saving the match, saving processing time.

How do lookahead and lookbehind assertions work?

Lookahead "(?=...)" checks if a specific pattern follows the current position, and lookbehind "(?<=...)" checks if a pattern precedes it. They are zero-width assertions, meaning they do not include the checked text in the match.

How do I escape special characters in a regex pattern?

To match literal special characters (like ., *, +, ?, ^, $, (, ), [, ], {, }, |, \), you must prefix them with a backslash (e.g. use "\." to match a literal period).

Why does my regex behave differently in JavaScript vs Python?

Different languages use different regex engines (e.g. JavaScript uses ECMA-262, Python uses its built-in re module). While core features are identical, syntax support for advanced lookbehinds and named capture groups varies.

What is a word boundary anchor (\b)?

A word boundary "\b" is a zero-width assertion that matches the boundary between a word character (letters, digits, underscore) and a non-word character or the start/end of the string.

How can I prevent ReDoS (Regular Expression Denial of Service)?

To prevent ReDoS, keep patterns simple, avoid nested quantifiers (like repeating groups that contain repeated characters), use timeout limits in your regex engine, and validate input lengths before running regex evaluations.

utility calculator

Regular Expression (Regex) Tester & Explainer

Test and debug regular expressions with real-time match highlighting, group capture breakdown, and a cheat sheet.

Formula Verified by Dr. Arthur Pendelton, PhD

Inputs

Interactive Regular Expression Tool

Regular Expression Pattern

Global (g)Case Insensitive (i)Multiline (m)

Test Subject String

Real-Time Highlights

Capture Group & Match Dashboard

Match #1 (idx: 14)

support@example.com

Match #2 (idx: 37)

sales@website.org

Common Regex Tokens Cheat Sheet

Any decimal digit [0-9]

Alphanumeric [a-zA-Z0-9_]

Whitespace (space, tab, newline)

Word boundary anchor

Start of subject string

End of subject string

One or more repetitions

Zero or more repetitions

Results

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utility calculator

Regular Expression (Regex) Tester & Explainer

Test and debug regular expressions with real-time match highlighting, group capture breakdown, and a cheat sheet.

Formula Verified by Dr. Arthur Pendelton, PhD

Inputs

Interactive Regular Expression Tool

Regular Expression Pattern

Global (g)Case Insensitive (i)Multiline (m)

Test Subject String

Real-Time Highlights

Capture Group & Match Dashboard

Match #1 (idx: 14)

support@example.com

Match #2 (idx: 37)

sales@website.org

Common Regex Tokens Cheat Sheet

Any decimal digit [0-9]

Alphanumeric [a-zA-Z0-9_]

Whitespace (space, tab, newline)

Word boundary anchor

Start of subject string

End of subject string

One or more repetitions

Zero or more repetitions

Results

Related guides

Learn more about this calculation

Guide

Understanding Calculator Formulas: How DTC Turns Inputs into Results

Understand how Do The Calculation formulas are presented, what the explanation blocks mean, and how to verify calculator logic before using a result in a real decision.

Site Guide

How to Use Do The Calculation Calculators: A Practical Step-by-Step Guide

Learn the fastest reliable workflow for using Do The Calculation calculators, reading results, checking formulas, and using save, print, share, and export actions correctly.