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

g enables global matching across all occurrences, i makes searches case-insensitive, m treats ^ and $ as line starts/ends, and s allows . to match newline characters.

How can I view captured parentheses groups?

When your pattern contains capture parentheses like (\w+), the match details panel displays each indexed group alongside the full match.

Can I test string replacements and redaction?

Yes. Enter a substitution value in the Replace field to preview how regex matches are transformed in real time.

Does this tool transmit my sample data across the network?

No. The regular expression runs purely in your local browser runtime. No strings or patterns are ever uploaded.

Regex Tester & Debugger

Runs in Browser

Test regular expressions in real-time with capture groups and string replacement.

How to Use the Regex Tester & Debugger

Enter your regular expression pattern in the pattern input bar.

Toggle flags such as Global (g), Case-insensitive (i), Multiline (m), or Dotall (s).

Type or paste your sample text into the Test String input container.

Inspect matched tokens, exact character start/end index positions, and extracted capture groups.

Optionally specify a replacement string to preview regex substitutions in real time.

Understanding Regex Tester & Debugger

Regular expressions (commonly abbreviated as Regex or RegExp) are algebraic character sequences that define search patterns for string matching, parsing, data validation, and text replacement. Supported in virtually every modern programming language—including JavaScript, Python, Go, Java, Rust, and PHP—regular expressions power everything from form validation (email addresses, phone numbers, UUIDs) to data wrangling and web scraping. However, crafting regular expressions can be notoriously error-prone due to nuanced syntax rules for quantifiers, greedy vs lazy evaluation, lookaheads, and capture group parentheses. A single missing escape character can invalidate a production deployment or create catastrophic backtracking loops. Our Regex Tester provides a developer-friendly workbench to test and inspect patterns with absolute confidence: 1. **Real-Time Validation & Execution:** Patterns are evaluated against your test text as you type, reporting exact execution durations in milliseconds. 2. **Clear Error Reporting:** In the event of an unclosed bracket or invalid quantifier, detailed syntax error messages guide immediate fixes. 3. **Capture Group Extraction:** Break down complex multi-group patterns. Each matched token displays its parent match along with individual parenthesized sub-group strings. 4. **Interactive Flag Controls:** Toggle global search (g), case-insensitivity (i), multi-line anchor matching (m), and dotall newline matching (s) with one click. 5. **Live Substitution Tool:** Test string replacements (such as redacting email addresses, stripping HTML tags, or formatting phone numbers) with instantaneous preview. All evaluation uses your browser's native JavaScript RegExp engine, providing 100% fidelity with production frontend behavior and keeping your confidential sample data strictly on your machine.