Regex Pattern Tester & Evaluator
Test JavaScript regular expressions with real-time matching, group captures, flags (g, i, m, s), and match count highlights.
Mastering Regular Expressions (Regex) in Web Engineering
Regular Expressions (Regex) are powerful sequences of characters that form a search pattern. They are used for string validation (e.g., verifying email syntax), string parsing (extracting capture groups), and string transformation across backend languages like Node.js, Python, and Rust.
Essential Regex Character Classes Quick Reference
\d: Matches any digit (equivalent to[0-9]).\w: Matches any word character (letters, digits, underscore).\s: Matches any whitespace character (space, tab, newline).+: Quantifier matching 1 or more occurrences.*: Quantifier matching 0 or more occurrences.?: Quantifier matching 0 or 1 occurrence (or ungreedy modifier).
How to Use Regex Pattern Tester & Evaluator
- Enter your regular expression pattern (e.g. [a-z0-9._%+-]+@[a-z0-9.-]+\.[a-z]{2,}) without surrounding slashes.
- Toggle flags such as Global (g), Case Insensitive (i), and Multiline (m).
- Type or paste test text into the Test String Input field.
- Real-time matches, capture group indices, and match counts are evaluated instantly.
Examples
Match Email Addresses
Contact support@codai.pro or lucky@university.edu for assistance.
[1] "support@codai.pro" at index 8
[2] "lucky@university.edu" at index 28
Frequently Asked Questions
What do the regex flags (g, i, m) mean?
Flags modify search behavior: `g` (global) finds all matches instead of stopping after the first match; `i` (case-insensitive) ignores letter casing; `m` (multiline) treats `^` and `$` as beginning and end of each line.
What is Catastrophic Backtracking in Regular Expressions?
Catastrophic backtracking (ReDoS) occurs when nested quantifiers (like `(a+)+`) cause an exponential number of search paths, freezing CPU execution. Always bound quantifiers in production regex.
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