Regex Tester & Validator
This tool helps you test, validate, and understand regular expressions by running them against sample text in real time. As you type or modify a pattern, matches update instantly, making it easier to see exactly what your expression captures and why.
In addition to matching, the tool explains the structure of the expression, highlights errors, and shows precise match locations. This makes it useful not only for debugging, but also for learning and refining complex patterns.
How Regex Testing Works
A regular expression is compiled using the selected flags and applied to the test string. If the pattern is invalid, the engine reports an error instead of attempting to match.
When the expression is valid, the engine scans the input text and records every match, along with its line and column positions. These results are displayed live, allowing you to adjust the pattern incrementally and observe how behavior changes.
Inputs and Options Explained
The tool exposes the most commonly used regular expression controls so you can fine-tune matching behavior without leaving the page.
- Regular expression — The pattern to test, written using standard regex syntax.
- Test string — The input text against which the expression is evaluated.
- Global (g) — Finds all matches instead of stopping after the first.
- Ignore case (i) — Makes matching case-insensitive.
- Multiline (m) — Changes how start and end anchors behave across lines.
- Dot matches newline (s) — Allows the dot character to match line breaks.
- Unicode (u) — Enables full Unicode character handling.
Examples and Edge Cases
Some expressions behave differently depending on flags. For example, a pattern that works on a single line may fail to match across multiple lines unless multiline or dot-all behavior is enabled.
Invalid patterns — such as unclosed character classes or mismatched parentheses — are caught immediately. This prevents silent failures and helps pinpoint syntax issues early.
Who Should Use This Tool
This tool is useful for anyone working with text processing, validation, or search patterns.
- Developers writing or debugging regex-based logic
- QA engineers validating input rules
- Data analysts cleaning or extracting text data
- Students learning how regular expressions work
Related Concepts
Understanding a few core regex ideas can make complex patterns much easier to reason about.
- Character classes — Sets of characters defined using square brackets.
- Quantifiers — Symbols that control how many times a token may repeat.
- Anchors — Tokens that match positions rather than characters.
When regex is used to validate or extract structured data, inspecting the results with the JSON Formatter can make debugging easier. For testing patterns against encoded strings, the URL Encoder & Decoder is often used alongside regex tools.