Regex Pattern Builder

Build, test, and debug regular expressions visually. Real-time match highlighting, pattern explanation, and code export. Everything runs in your browser.

Regex Pattern Use JavaScript regex syntax

/ / g

g global i case-insensitive m multiline s dotall u unicode

Common Patterns

Test String

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Pattern Explanation

Code Export

Regex Quick Reference

Character Classes

                                . Any character (except newline)

                                \d Digit [0-9]

                                \D Non-digit

                                \w Word char [a-zA-Z0-9_]

                                \W Non-word char

                                \s Whitespace

                                \S Non-whitespace

                                [abc] Character set

                                [^abc] Negated set

Quantifiers

                                * 0 or more

                                + 1 or more

                                ? 0 or 1

                                {n} Exactly n

                                {n,} n or more

                                {n,m} Between n and m

                                *? Lazy 0 or more

                                +? Lazy 1 or more

Anchors & Boundaries

                                ^ Start of string/line

                                $ End of string/line

                                \b Word boundary

                                \B Non-word boundary

Groups

                                (abc) Capturing group

                                (?:abc) Non-capturing

                                (?<name>abc) Named group

                                a|b Alternation

Lookaround

                                (?=abc) Positive lookahead

                                (?!abc) Negative lookahead

                                (?<=abc) Positive lookbehind

                                (?<!abc) Negative lookbehind

Frequently Asked Questions

What is a regular expression (regex)?▼

A regular expression is a sequence of characters that defines a search pattern. Regex is used for string matching, validation, search-and-replace, and text extraction in virtually every programming language.

What do regex flags like g, i, and m mean?▼

The 'g' (global) flag finds all matches instead of just the first. The 'i' flag makes matching case-insensitive. The 'm' (multiline) flag makes ^ and $ match the start/end of each line rather than the entire string. The 's' (dotall) flag makes . match newline characters. The 'u' (unicode) flag enables full Unicode matching.

How do capturing groups work in regex?▼

Capturing groups are created with parentheses (). They capture the matched text so you can reference it later. For example, (\d{3})-(\d{4}) would capture the area code and number separately. Named groups use (?<name>...) syntax. Non-capturing groups (?:...) group without capturing.

What is the difference between greedy and lazy quantifiers?▼

Greedy quantifiers (*, +, {n,m}) match as much text as possible, while lazy quantifiers (*?, +?, {n,m}?) match as little as possible. For example, given '<b>bold</b>', the greedy pattern <.*> matches the entire string, while <.*?> matches just '<b>' and '</b>' separately.

How do lookahead and lookbehind assertions work?▼

Lookahead (?=...) and lookbehind (?<=...) are zero-width assertions that check for patterns without including them in the match. Positive lookahead (?=...) asserts what follows, negative lookahead (?!...) asserts what must not follow. Similarly, (?<=...) and (?<!...) work for lookbehind.

Can I use this regex builder offline?▼

Yes! DevToolbox is a Progressive Web App. Once you visit the page, it is cached by the service worker and works fully offline. All regex processing happens in your browser using JavaScript's built-in RegExp engine -- no data is sent to any server.

Regex Pattern Builder

Character Classes

Quantifiers

Anchors & Boundaries

Groups

Lookaround

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Frequently Asked Questions