Validate input rules
Test patterns for emails, slugs, employee IDs, or order references before you wire them into app validation logic.
Regex Tester
브라우저 내부 처리
Test regex patterns, flags, and replacements against your input text.
Where a regex tester saves real time
Extract values from logs
Build patterns that pull UUIDs, request IDs, paths, or token-like strings out of noisy incident logs.
Preview replacements safely
Check what a pattern really matches before doing large-scale replacements or text cleanup in another tool or codebase.
Things that trip people up a lot
What do g, i, and m change?
g, i, and m change?g searches globally, i ignores case, and m changes how line anchors behave across multiple lines.
Why do regex syntax errors happen so often?
Missing escapes, unbalanced groups, or unsupported syntax are the common causes. Testing a smaller fragment first is usually the fastest fix.
Which regex engine does this page use?
It uses the browser JavaScript regex engine. That means behavior can differ from PCRE or some server runtimes, so production checks still matter.
Working with regular expressions
Greedy quantifiers take more than you expect
.* matches as much as possible, so .* spans the whole string rather than one tag. Use .*? to match minimally. When results look wrong on tags or quotes, check this first.
Syntax differs between engines
JavaScript, Python, PCRE and POSIX diverge on lookbehind and Unicode property support. An expression that works here may not work in your server language. Re-test it in that runtime when you move it.
Large input can hang the engine
Nested quantifiers such as (a+)+ cause catastrophic backtracking and stall on short input. In code validating user input this becomes a denial of service. Splitting complex patterns into string operations is safer.