How to Use Regex Tester

1. Enter your regular expression pattern in the input field without leading or trailing slashes.
2. Configure regex flags: Global (g), Case-Insensitive (i), Multiline (m), DotAll (s), and Unicode (u).
3. Type or paste your test subject text into the subject textarea.
4. Inspect highlighted matches in the visual highlight viewer and check capture group extractions in the match breakdown drawer.
5. Switch to the "Find & Replace" tab to preview string substitutions using $1, $2, or

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   template variables in real-time.
6. Click "Export Code" to generate ready-to-run snippets in JavaScript/TypeScript, Python, Go, or PHP.

Frequently Asked Questions

What is a regular expression (Regex) and how does it work?

A regular expression is a specialized sequence of characters that defines a search pattern. A regex engine compiles the pattern into a state machine (deterministic or non-deterministic finite automaton) that scans the target string to locate matching substrings, validate input formats, or extract delimited capture groups.

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

Regex flags modify search behavior: Global (g) matches all occurrences across the subject string instead of stopping at the first match; Insensitive (i) makes letter matching case-insensitive; Multiline (m) allows ^ and $ to match the beginning and end of each line; DotAll (s) allows the dot (.) character to match newline characters (\n); and Unicode (u) treats the pattern as a sequence of full UTF-16 code points.

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

A capturing group `(pattern)` isolates matched text and assigns it an index ($1, $2, etc.) or a named variable, allowing programmatic extraction or replacement. A non-capturing group `(?:pattern)` applies quantifiers or alternation to sub-expressions without memory allocation or back-reference capture, which improves matching speed and reduces memory overhead.

How do lookahead and lookbehind assertions work?

Lookaheads and lookbehinds are zero-width assertions that verify conditions without consuming characters in the match result. Positive lookahead `(?=abc)` verifies that "abc" follows the current position. Negative lookahead `(?!abc)` ensures "abc" does not follow. Positive lookbehind `(?<=abc)` verifies that "abc" precedes the position, and negative lookbehind `(?

How do I substitute text using capture groups ($1, $2)?

In the replacement string, `$1`, `$2`, and `$n` reference the corresponding capture group indexed from left to right. `

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` inserts the entire matched substring, while ` ` and ` ` and `

` insert the text preceding and following the match respectively. For example, replacing `(\\w+)\\s(\\w+)` with `$2, $1` transforms \"John Doe\" into \"Doe, John\"." } }, { "@type": "Question", "name": "What is Catastrophic Backtracking (ReDoS)?", "acceptedAnswer": { "@type": "Answer", "text": "Regular Expression Denial of Service (ReDoS) occurs when nested quantifiers (such as `(a+)+ ) cause the regex engine to explore exponentially many permutations when attempting to match non-matching strings. Toolique safeguards your browser by enforcing execution safety limits and tracking execution time in milliseconds." } }, { "@type": "Question", "name": "What are standard regular expressions for Indian identification and billing?", "acceptedAnswer": { "@type": "Answer", "text": "Indian business and government compliance patterns: PAN Card: `^[A-Z]{5}[0-9]{4}[A-Z]{1} ; GSTIN: `^\\d{2}[A-Z]{5}\\d{4}[A-Z]{1}[1-9A-Z]{1}Z[0-9A-Z]{1} ; Indian 10-digit mobile: `^(?:\\+91|91|0)?[6-9]\\d{9} ; Indian PIN Code: `^[1-9][0-9]{5} ; Formatted Aadhaar: `^[2-9]{1}[0-9]{3}\\s[0-9]{4}\\s[0-9]{4} ." } } ] }, { "@type": "Organization", "@id": "https://www.toolique.in/#organization", "name": "Toolique", "url": "https://www.toolique.in", "logo": "https://www.toolique.in/favicon-512x512.png", "foundingDate": "2026-06-16", "description": "Toolique is a premium, free, privacy-focused online suite of interactive calculators, developer converters, and programming academy tools operating completely in the browser.", "contactPoint": { "@type": "ContactPoint", "contactType": "customer support", "email": "support@toolique.in" }, "founder": { "@type": "Person", "@id": "https://www.toolique.in/about-founder#person" }, "sameAs": [ "https://github.com/ajinkyaswami1999", "https://www.linkedin.com/in/ajinkya-swami-82751b191/", "https://www.instagram.com/ajinkyaswami.in/", "https://voxelique.com", "https://www.instagram.com/voxelique/" ] }, { "@type": "Person", "@id": "https://www.toolique.in/about-founder#person", "name": "Ajinkya Swami", "jobTitle": "Founder & Software Architect", "image": "https://www.toolique.in/favicon-512x512.png", "description": "Ajinkya Swami is a QA Automation Engineer, Full-Stack Developer, and founder of Toolique and Voxelique, specializing in custom desktop automation script systems and web application development.", "worksFor": { "@type": "Organization", "@id": "https://www.toolique.in/#organization" }, "founderOf": [ { "@type": "Organization", "@id": "https://www.toolique.in/#organization" } ], "knowsAbout": [ "Software QA Verification", "QA Automation Frameworks", "Python & Selenium Models", "React & Next.js Stacks", "3D Printing Custom Design Solutions" ], "sameAs": [ "https://github.com/ajinkyaswami1999", "https://www.linkedin.com/in/ajinkya-swami-82751b191/", "https://www.instagram.com/ajinkyaswami.in/", "https://voxelique.com", "https://www.instagram.com/voxelique/" ] }, { "@type": "WebSite", "@id": "https://www.toolique.in/#website", "name": "Toolique", "url": "https://www.toolique.in", "publisher": { "@type": "Organization", "@id": "https://www.toolique.in/#organization" }, "potentialAction": { "@type": "SearchAction", "target": "https://www.toolique.in/tools?q={search_term_string}", "query-input": "required name=search_term_string" } } ] }

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` insert the text preceding and following the match respectively. For example, replacing `(\w+)\s(\w+)` with `$2, $1` transforms "John Doe" into "Doe, John".

What is Catastrophic Backtracking (ReDoS)?

Regular Expression Denial of Service (ReDoS) occurs when nested quantifiers (such as `(a+)+ ` and ` ) cause the regex engine to explore exponentially many permutations when attempting to match non-matching strings. Toolique safeguards your browser by enforcing execution safety limits and tracking execution time in milliseconds.

What are standard regular expressions for Indian identification and billing?

Indian business and government compliance patterns: PAN Card: `^[A-Z]{5}[0-9]{4}[A-Z]{1} ` and ` ; GSTIN: `^\d{2}[A-Z]{5}\d{4}[A-Z]{1}[1-9A-Z]{1}Z[0-9A-Z]{1} ` and ` ; Indian 10-digit mobile: `^(?:\+91|91|0)?[6-9]\d{9} ` and ` ; Indian PIN Code: `^[1-9][0-9]{5} ` and ` ; Formatted Aadhaar: `^[2-9]{1}[0-9]{3}\s[0-9]{4}\s[0-9]{4} ` and ` .