What is a regular expression (regex) and how do I use it?

A regular expression (regex) is a sequence of characters that defines a search pattern for matching text. It's used for pattern matching, text validation, search and replace, and data extraction. Common uses include email validation, phone number formatting, URL parsing, and log file analysis.

How do I test a regex pattern online?

Enter your regex pattern in the pattern field, paste your test string in the input area, select the appropriate flags (g, i, m, s, u), and see matches highlighted in real-time. The tool shows all matches, capture groups, and generates code for JavaScript, Python, and PHP.

What do the regex flags g, i, m, s, u, y mean?

g (global) finds all matches, not just the first. i (case-insensitive) ignores uppercase/lowercase. m (multiline) makes ^ and $ match line boundaries. s (dotAll) makes . match newlines. u (unicode) enables full unicode support. y (sticky) matches only from lastIndex position.

How do I match a literal special character in regex?

Escape special characters with a backslash (\). For example: \. matches a literal dot, \* matches asterisk, \[ matches opening bracket, \( matches parenthesis, \$ matches dollar sign. Special characters include: . * + ? ^ $ { } [ ] \ | ( )

What is the difference between * and + quantifiers in regex?

* (asterisk) matches zero or more occurrences - the pattern can match nothing. + (plus) matches one or more occurrences - requires at least one match. Example: a* matches "", "a", "aaa" while a+ matches "a", "aaa" but not empty string.

How do capture groups work in regular expressions?

Parentheses () create capture groups that extract matched substrings. Named groups (? ...) allow referencing by name. Non-capturing groups (?:...) group without capturing. Access groups via match[1], match[2] or match.groups.name in code.

What are lookahead and lookbehind assertions in regex?

Lookahead (?=...) checks what follows without consuming characters. Negative lookahead (?!...) ensures pattern doesn't follow. Lookbehind (?<=...) checks what precedes. Negative lookbehind (?<!...) ensures pattern doesn't precede. They're zero-width assertions for complex matching.

How do I match the start or end of a string in regex?

^ matches the start of a string (or line with m flag). $ matches the end of a string (or line with m flag). \b matches word boundaries. \B matches non-word boundaries. Example: ^Hello matches "Hello World" but not "Say Hello".

What is the difference between greedy and lazy matching?

Greedy quantifiers (*, +, ?) match as much as possible. Lazy/non-greedy quantifiers (*?, +?, ??) match as little as possible. Example: for " text ", matches the entire string (greedy), while matches just " " (lazy).

How do I match any character including newlines?

The dot . matches any character except newline by default. With the s (dotAll) flag, . matches newlines too. Alternative: [\s\S] or [\d\D] matches any character including newlines without the s flag.

How do I validate an email address with regex?

A basic email regex: ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$ This matches most valid emails. For production, consider using built-in email validation or more comprehensive patterns that handle edge cases.

How do I extract numbers from text using regex?

Use \d+ to match one or more digits. For decimals: \d+\.?\d* or \d+(\.\d+)? For negative numbers: -?\d+ For currency: \$?\d{1,3}(,\d{3})*(\.\d{2})? The tool shows all matches and capture groups.

Can I test my regex in different programming languages?

Yes! This tool generates equivalent code for JavaScript, Python, and PHP. Note that some advanced features like lookbehind have varying support. JavaScript uses /pattern/flags, Python uses re.compile(), PHP uses preg_match().

What are character classes in regex?

Character classes match specific character sets: \d matches digits (0-9), \w matches word characters (a-z, A-Z, 0-9, _), \s matches whitespace. Uppercase versions (\D, \W, \S) match the opposite. Custom classes: [abc] matches a, b, or c.

Is my regex data secure when using this online tester?

Yes! All regex testing happens 100% in your browser using JavaScript. Your patterns and test strings are NEVER sent to any server, stored, or logged. The tool works completely offline for maximum privacy and security.

What is a regular expression (regex) and how do I use it?

A regular expression (regex) is a sequence of characters that defines a search pattern for matching text. It's used for pattern matching, text validation, search and replace, and data extraction. Common uses include email validation, phone number formatting, URL parsing, and log file analysis.

How do I test a regex pattern online?

Enter your regex pattern in the pattern field, paste your test string in the input area, select the appropriate flags (g, i, m, s, u), and see matches highlighted in real-time. The tool shows all matches, capture groups, and generates code for JavaScript, Python, and PHP.

What do the regex flags g, i, m, s, u, y mean?

g (global) finds all matches, not just the first. i (case-insensitive) ignores uppercase/lowercase. m (multiline) makes ^ and $ match line boundaries. s (dotAll) makes . match newlines. u (unicode) enables full unicode support. y (sticky) matches only from lastIndex position.

How do I match a literal special character in regex?

Escape special characters with a backslash (\). For example: \. matches a literal dot, \* matches asterisk, \[ matches opening bracket, \( matches parenthesis, \$ matches dollar sign. Special characters include: . * + ? ^ $ { } [ ] \ | ( )

What is the difference between * and + quantifiers in regex?

* (asterisk) matches zero or more occurrences - the pattern can match nothing. + (plus) matches one or more occurrences - requires at least one match. Example: a* matches "", "a", "aaa" while a+ matches "a", "aaa" but not empty string.

How do capture groups work in regular expressions?

Parentheses () create capture groups that extract matched substrings. Named groups (? ...) allow referencing by name. Non-capturing groups (?:...) group without capturing. Access groups via match[1], match[2] or match.groups.name in code.

What are lookahead and lookbehind assertions in regex?

Lookahead (?=...) checks what follows without consuming characters. Negative lookahead (?!...) ensures pattern doesn't follow. Lookbehind (?<=...) checks what precedes. Negative lookbehind (?<!...) ensures pattern doesn't precede. They're zero-width assertions for complex matching.

How do I match the start or end of a string in regex?

^ matches the start of a string (or line with m flag). $ matches the end of a string (or line with m flag). \b matches word boundaries. \B matches non-word boundaries. Example: ^Hello matches "Hello World" but not "Say Hello".

What is the difference between greedy and lazy matching?

Greedy quantifiers (*, +, ?) match as much as possible. Lazy/non-greedy quantifiers (*?, +?, ??) match as little as possible. Example: for " text ", matches the entire string (greedy), while matches just " " (lazy).

How do I match any character including newlines?

The dot . matches any character except newline by default. With the s (dotAll) flag, . matches newlines too. Alternative: [\s\S] or [\d\D] matches any character including newlines without the s flag.

How do I validate an email address with regex?

A basic email regex: ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$ This matches most valid emails. For production, consider using built-in email validation or more comprehensive patterns that handle edge cases.

How do I extract numbers from text using regex?

Use \d+ to match one or more digits. For decimals: \d+\.?\d* or \d+(\.\d+)? For negative numbers: -?\d+ For currency: \$?\d{1,3}(,\d{3})*(\.\d{2})? The tool shows all matches and capture groups.

Can I test my regex in different programming languages?

Yes! This tool generates equivalent code for JavaScript, Python, and PHP. Note that some advanced features like lookbehind have varying support. JavaScript uses /pattern/flags, Python uses re.compile(), PHP uses preg_match().

What are character classes in regex?

Character classes match specific character sets: \d matches digits (0-9), \w matches word characters (a-z, A-Z, 0-9, _), \s matches whitespace. Uppercase versions (\D, \W, \S) match the opposite. Custom classes: [abc] matches a, b, or c.

Is my regex data secure when using this online tester?

Yes! All regex testing happens 100% in your browser using JavaScript. Your patterns and test strings are NEVER sent to any server, stored, or logged. The tool works completely offline for maximum privacy and security.

</>ToolForge

Maintenance in progress — Some tools may not function properly. We apologize for any inconvenience.

Regex Tester & Builder

medium

Test and build regular expressions with live matching, capture groups, and code generation for JavaScript, Python, and PHP.

Web Developmentregexregular expressionpatternmatch

100% Private - Runs Locally

How to Use

1Enter regex pattern
2Add test string
3Toggle flags (g, i, m)
4View matches and groups

Examples

Email Validation

Input

Pattern: [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}
Text: Contact us at support@example.com or sales@company.org

Output

Matches: support@example.com, sales@company.org

Extract Phone Numbers

Input

Pattern: \d{3}-\d{3}-\d{4}
Text: Call 555-123-4567 or 800-555-0199

Output

Matches: 555-123-4567, 800-555-0199

Named Capture Groups

Input

Pattern: (?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})
Text: Date: 2024-12-21

Output

Match: 2024-12-21
Groups: year=2024, month=12, day=21

Frequently Asked Questions

Related Tools

View all

Text Diff Checker

Compare two texts and highlight differences line-by-line and character-by-character. Find changes between code versions, document revisions, and configuration files with unified diff output.

String Manipulation

Comprehensive string manipulation toolkit: trim whitespace, pad strings, truncate text, slugify URLs, escape/unescape characters, extract substrings, and perform 15+ text operations for developers.