What this tool does
Regex Tester
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Keep regex pages practical and debugging-oriented so they serve immediate pattern-testing intent.
How to use this tool
How to use Regex Tester
Enter a pattern.
Paste test text.
Run the tester and review matches.
Before and after example
Regex Tester example
Input
\bscript_\w+\bOutput
{
"matches": [{"match": "script_tools"}],
"match_count": 1
}Options, errors, and limitations
Regex Tester options and common errors
Parent category
Explore the Security Tools hub
Developer utilities for JWT inspection, hashing, regex review, and safe encoding workflows.
Related articles
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FAQ
Regex Tester FAQ
Which regex flavor does an online tester use?
Regex syntax differs across JavaScript, Python, PCRE, Java, .NET, and other engines. Always confirm the flavor because lookbehind, named groups, Unicode classes, flags, and escape rules are not identical everywhere.
What is the difference between multiline and dotall modes?
Multiline mode usually changes how the start and end anchors behave around line boundaries, while dotall changes whether the dot character can match newline characters. The exact flag names depend on the regex engine.
Why does dot not match a newline?
In many regex engines, dot matches most characters except line terminators by default. Enable the engine's dotall mode or use an explicit character class when you intentionally need matches to cross lines.
What does the global flag do?
In engines such as JavaScript, the global flag makes the operation continue finding later matches instead of stopping after the first. It does not change the pattern itself, but it changes how repeated matching is performed.
Why does lookbehind work in one browser or language but not another?
Regex feature support depends on the engine version. Modern environments increasingly support lookbehind, but older browsers and some engines may reject the syntax entirely.
Can a regex cause performance or denial of service problems?
Yes. Certain nested quantifiers and ambiguous patterns can produce catastrophic backtracking on carefully chosen input. Security sensitive applications should review complex user facing regexes and enforce practical input limits.
Why do I need to escape a backslash twice in code?
The regex parser and the programming language string parser may both interpret backslashes. A regex written as a source literal can therefore require different escaping than the same pattern typed directly into a tester.
How does Unicode affect regex matching?
ASCII oriented classes such as \w may not match every letter or word character from other languages. Use Unicode aware flags or property escapes when the target engine supports them and international text matters.
What are capture groups and noncapture groups?
Capture groups store the text matched by a subexpression so it can be referenced later, while noncapture groups group pattern logic without creating a captured result. Reducing unnecessary captures can make complex patterns clearer.
Is a regex tester enough to prove a pattern is correct?
No. Test normal cases, edge cases, empty input, long input, Unicode, invalid data, and adversarial strings. A few successful examples do not prove the pattern is complete or safe for production.