Last updated
Validate regular expressions with custom flags and real sample text.
Quick answer: Regex Tester
Live-match your pattern against sample text - see captures without shipping to prod.
Example: Pattern `^\d{3}-\d{4}$` on `555-0199` → match; on `(555) 0199` → fail - reveals you need normalization before validation.
Calculations follow the documented formula on this page; rounding and input units can change the last digit-treat outputs as educational estimates unless you reconcile with source systems.
* This is an estimate. Actual amounts may vary slightly based on input assumptions.
Dialects differ: PCRE vs JavaScript vs POSIX. This tool mirrors your stack’s flavor or you’ll debug ghosts. Greedy vs lazy quantifiers still trip seniors.
Validate regular expressions with custom flags and real sample text.
Inputs on this page: Regex Pattern, Test Text, Flags. Assumptions stay visible so you can reproduce the figure elsewhere.
Long-form walkthroughs that pair well with this calculator. When you need narrative context beyond the live fields, start here and return to the tool to plug in your own numbers.
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Continue in the Developer category hub, the Developer tools collection, or the glossary. Related calculators in this session: Word Counter, JSON Validator, Base64 Encoder/Decoder, URL Encoder/Decoder, API Response Formatter, JSON Formatter.
The Formula
Run regex pattern with flags and return match results| This tool | Run regex pattern with flags and return match results |
|---|---|
| Related intent: regex tester | See paired tools for regex tester-each page documents its own core relationship next to the live form. |
| Related intent: regex tester online usa | See paired tools for regex tester online usa-each page documents its own core relationship next to the live form. |
Core relationship for Regex Tester:
Run regex pattern with flags and return match results
Worked check: Pattern `^\d{3}-\d{4}$` on `555-0199` → match; on `(555) 0199` → fail - reveals you need normalization before validation.
Keep the same assumptions and open a neighbor calculator when your question branches: Word Counter, JSON Validator, Base64 Encoder/Decoder, URL Encoder/Decoder. Each page documents its own formula beside the fields.
Learning links: Methodology · Editorial policy · Glossary
Pattern `^\d{3}-\d{4}$` on `555-0199` → match; on `(555) 0199` → fail - reveals you need normalization before validation.
Re-enter the same numbers in the calculator above to confirm the page math matches the interactive result.
People who care about regex tester use Regex Tester as a planning sandbox: Never regex-parse email for production - use a library and allow IDN.
Instant response
Run Regex Tester in the browser and read the breakdown beside the form.
Transparent formula
The formula and worked example on this page match what the calculator uses.
Privacy friendly
No account required; inputs stay in your session unless you choose to share them.
Cross-device ready
Layout works on mobile, tablet, and desktop for the same field labels.
Official references for context. Calculator outputs are planning estimates—confirm material decisions with the primary authority or a qualified professional. See our methodology and editorial policy.
Reviewed July 18, 2026 · Content stamp 2026-07-18
Click a question to expand the answer.
Use standard flags such as g, i, and m based on your matching needs.
Yes, syntax errors and match output help you iterate quickly.
Check anchors, escaping, and flags since small pattern issues can prevent matches.
Yes, provide multiline input and use relevant flags as needed.
Yes, it is useful for building and verifying form/input regex rules.
Live-match your pattern against sample text - see captures without shipping to prod.
Pattern `^\d{3}-\d{4}$` on `555-0199` → match; on `(555) 0199` → fail - reveals you need normalization before validation.
Never regex-parse email for production - use a library and allow IDN. Catastrophic backtracking is real - test with evil strings. Multiline mode changes ^ and $ behavior - toggle intentionally.
Same-category picks first, then high-intent neighbors that often answer the next calculation question.
JSON Formatter
Format minified JSON beautifully.
JSON Validator
Validate JSON syntax instantly with error feedback.
Base64 Encoder/Decoder
Encode or decode Base64 text for APIs and integrations.
URL Encoder/Decoder
Encode or decode URL strings for query and path safety.
API Response Formatter
Format and validate API JSON responses for readability.
Schema Markup Generator
Generate valid JSON-LD schema blocks for common page types.
Open the full directory, browse your hub collection, or jump back to this category. Bookmark the page if you use it often.
Reviewed by Imtiaz Ahmad
Founder & lead engineer | Last reviewed July 18, 2026
See methodology and editorial policy.