What Is Regex?
Regular expressions (regex or regexp) are sequences of characters that define search patterns. They are a fundamental tool in programming for string searching, data validation, text extraction, and find-and-replace operations. Every major language — JavaScript, Python, Java, PHP, Go, Ruby — has native regex support.
A regex pattern like \d3-\d4 matches a phone segment with 3 digits, a hyphen, and 4 digits. Patterns can range from simple literals to complex expressions with lookaheads, named groups, and Unicode properties.
How to test a regular expression
Step 1
Enter a pattern
Type only the pattern body. The surrounding slashes are shown by the interface and are not part of the input.
Step 2
Choose flags
Turn on global, case-insensitive, multiline, dotAll, Unicode, or sticky behavior as required.
Step 3
Inspect and replace
Review match indexes and groups, then test JavaScript replacement tokens before copying or exporting results.
Regex Accuracy and Performance Notes
This tester uses the JavaScript RegExp engine in your browser. Syntax and features can differ from PCRE, Python, Java, .NET, or RE2. Presets are practical examples, not a substitute for standards-aware parsing or business-rule validation. For example, the card-shape preset checks formatting only; it does not perform a Luhn check or confirm that a card exists.
Some patterns with nested or ambiguous repetition can cause excessive backtracking. Test unfamiliar patterns on small, non-sensitive samples before using them with large or untrusted input.
Regex Flags Reference
| Flag | Name | Effect |
|---|---|---|
| g | Global | Find all matches |
| i | Ignore Case | Case insensitive |
| m | Multiline | ^ $ match line ends |
| s | Dot All | Dot matches newline |
| u | Unicode | Full Unicode support |
| y | Sticky | Match from lastIndex |
Continue Your Developer Workflow
Clean and inspect structured data with the JSON Formatter & Validator, compare transformed output with the Text Compare & Diff Checker, prepare URL values with the URL Encoder & Decoder, or convert tabular test data using the CSV ↔ JSON Converter.
Technical References
Review the MDN JavaScript RegExp reference for practical syntax and browser guidance, or consult the ECMAScript regular-expression specification for normative engine behavior. External references open in a new tab.