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Regex Tester
Test regular expressions instantly against text input. Validate patterns, find matches, and debug regex expressions online.
Regular Expressions (RegEx) define formal search patterns using deterministic and non-deterministic finite automata under the ECMAScript RegExp standard. Live pattern testing allows developers to test character classes, quantifiers, lookahead/lookbehind assertions, and flag modifiers (`g`, `i`, `m`, `s`, `u`) before deploying to production code.
Common flags: g (global), i (ignore case), m (multiline)
Common Regex Examples
- Email:
^[^s@]+@[^s@]+.[^s@]+$ - Numbers:
^[0-9]+$ - Letters:
^[A-Za-z]+$ - Username:
^[a-zA-Z0-9_]{3,20}$ - Strong Password:
^(?=.*[A-Z])(?=.*[0-9])(?=.*[^A-Za-z0-9]).{8,}$
Language Specification StandardReviewed: January 2026
ECMAScript 2026 (ECMA-262) Regular Expression Standards
NFA Engine with Lookaround & Named Capture GroupsModern JavaScript regex engines support positive/negative lookaheads (`(?=...)`, `(?!...)`), lookbehinds (`(?<=...)`, `(?<!...)`), named capture groups (`(?<name>...)`), and Unicode property escapes (`\p{Letter}`). Testing protects against Regular Expression Denial of Service (ReDoS) caused by nested quantifiers.
Reference: ECMA International (ECMA-262) & Mozilla Developer Network (MDN)
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How the Regex Tester Works
1
Enter Regex Pattern
Type or paste the regular expression pattern.
2
Enter Test String
Provide the text you want to test against the regex.
3
Run Test
Instantly view matches and validation results.
4
Analyze Results
See matched text, match count, and validation status.
Regex Pattern Concept
/pattern/flags
Regular expressions are patterns used to match, validate, and manipulate text. They are widely used in programming, validation, search, and parsing tasks.
Example Calculation
Input: Pattern: ^[a-zA-Z]+$ | Text: DevCalc
Output: Match Found
Common Uses
- • Email Validation
- • Password Validation
- • Form Validation
- • Data Extraction
- • Search Operations
- • Log Analysis
- • Input Validation
- • Programming
Questions about this calculator
A regular expression is a sequence of characters that defines a search pattern used for matching, validating, and manipulating text.
A Regex Tester allows developers to test regular expressions against sample text and instantly see matching results.
Regular expressions are commonly used for validation, searching, filtering, extracting, and transforming text data.
Yes. Regular expressions are commonly used to validate email formats before form submission.
Yes. Regex patterns can enforce password rules such as minimum length, uppercase letters, numbers, and special characters.
Flags modify regex behavior. Examples include g (global), i (case-insensitive), and m (multiline).
A capture group stores matched portions of text so they can be reused or extracted later.
Common reasons include incorrect syntax, missing escape characters, invalid flags, or unexpected input data.
What Is a Regex Tester?
A Regex Tester is a developer tool used to validate and test regular expressions against sample text. Regular expressions help developers search, match, validate, and manipulate text efficiently.
What Are Regular Expressions?
Regular expressions (Regex) are text patterns used to identify specific combinations of characters within strings.
Common Regex Use Cases
- Email validation
- Password validation
- Phone number validation
- URL validation
- Data extraction
- Log file analysis
- Form validation
- Search and replace
Popular Regex Examples
| Purpose | Regex |
|---|---|
| ^[^\s@]+@[^\s@]+\.[^\s@]+$ | |
| Numbers Only | ^[0-9]+$ |
| Letters Only | ^[A-Za-z]+$ |
| Password | ^(?=.*[A-Z])(?=.*[0-9]).{8,}$ |
Why Use Our Regex Tester?
Our free Regex Tester allows developers to instantly test patterns, debug matching issues, and validate user input directly in the browser.
Learn more
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Written & maintained by Aditya Singh
Aditya Singh is a software engineer and the founder of DevCalc, based in Uttar Pradesh, India. He builds and maintains every tool on this site with careful attention to accuracy and user experience. Read more about the author →
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