Query guide: javascript regex flags tester
JavaScript regex flags tester for checking expression behavior online
Use this JavaScript regex flags tester when a pattern changes after adding g, i, m, s, u, or y. Check the expression with realistic samples, compare match ranges, and copy the final pattern with the exact flags your code will use.
JavaScript regex flags can change more than the number of matches. They affect case handling, line anchors, whether dot crosses newlines, Unicode property escapes, sticky parser behavior, and replacement scope. A pattern that looks correct in one short sample can fail when the same expression is run against multiline logs, form input, names, or parser streams.
Searches for check regex javascript, javascript regex checker, and regex tester expressions often lead to a flag decision. The safest workflow is to start with the smallest expression, turn on one flag at a time, and verify the result against pass and fail samples.
Treat flags as part of the expression contract. When the regex moves into browser, Node.js, or TypeScript code, the pattern body and the flag string should be copied and reviewed together.
How to test JavaScript regex flags online
- Run the expression with no optional flags first so the baseline match is visible.
- Turn on one JavaScript regex flag at a time and watch which match ranges, groups, and replacements change.
- Use `g` for extraction and replacement workflows, but turn it off when checking a simple boolean validator with `test()`.
- Retest multiline samples when `^`, `$`, `.`, or pasted line breaks are part of the bug.
- Copy the final pattern and flags together, then keep the same sample input beside the production code.
JavaScript regex flag reference
global
Find every match in the sample instead of stopping after the first one.
Compare one highlighted match with a full list of repeated matches.
case insensitive
Match uppercase and lowercase letters without adding both versions to the expression.
Check values like Error, ERROR, and error before copying the RegExp.
multiline
Make `^` and `$` work at the start and end of each pasted line.
Paste logs, textarea content, or CSV rows and test anchors line by line.
dotAll
Allow `.` to cross line breaks when a match must span multiple lines.
Use a sample with an intentional newline inside the value you expect to match.
unicode
Use Unicode-aware matching, property escapes, and safer handling for non-ASCII text.
Retest names, symbols, emoji, and property escapes such as `\p{Letter}`.
sticky
Match only at the current `lastIndex` position in a tokenizer or parser loop.
Use a step-by-step parser sample and confirm each token starts exactly where expected.
Starter expressions for flag checks
Case-insensitive log search
/^error:.*timeout$/gim
Sample: Error: request timeout
`i` accepts Error or error, while `m` lets anchors check each log line.
Match a block across lines
/BEGIN.*END/gs
Sample: BEGIN\nline one\nEND
`s` lets dot cross newline characters, and `g` finds repeated blocks.
Unicode letters only
/^\p{Letter}+$/u
Sample: Miyazaki, Saoirse, Zoë
`u` is required for Unicode property escapes in JavaScript.
Sticky tokenizer step
/\s*(?<word>[A-Za-z]+)/y
Sample: alpha beta gamma
`y` is useful when parser code advances from one exact index to the next.
Flag bugs worth catching before code ships
Flag mistakes are easy to miss because the pattern text still looks unchanged. Use the tester to prove which flag changed the behavior before the expression is copied into a validator, parser, replacement, or search feature.
- A validator passes once and fails next time because a reused global or sticky RegExp changed `lastIndex`.
- A pattern works on one line but misses pasted text because `m` or `s` was not tested with real line breaks.
- A JavaScript string copy loses backslashes even though the online pattern and flags were correct.
- Unicode names or symbols fail because property escapes were tested without the `u` flag.
- Replacement output changes more than expected because `g` replaces every match instead of only the first match.
Check JavaScript flags now
Open the live tester with a multiline log search already loaded. Toggle i and m to see how case and line anchors change the result.
Test Regex FlagsRelated Pages on Regex Tester
Open the live JavaScript regex flags tester
Toggle flags, inspect highlighted matches, check groups, and preview replacement output.
JavaScript regex tester
Use the broader RegExp workflow for flags, matches, groups, and replacements.
JavaScript regex checker
Check a pattern with the same flags your browser or Node.js code will use.
JavaScript regex validator
Validate full input values and avoid global flag surprises with boolean checks.
Regex debugger
Isolate a failing sample by changing anchors, groups, quantifiers, and flags one at a time.
Regex flags cheat sheet
Reference JavaScript flag meanings while building or reviewing an expression.
JavaScript Regex Flags FAQ
What is a JavaScript regex flags tester?
A JavaScript regex flags tester checks how `g`, `i`, `m`, `s`, `u`, and `y` change matches, groups, anchors, Unicode behavior, and replacement output before the RegExp is copied into code.
Which JavaScript regex flag should I test first?
Start without optional flags, then add the one flag tied to the behavior you need. Use `g` for all matches, `i` for case-insensitive matching, `m` for line anchors, `s` for dot across newlines, `u` for Unicode mode, and `y` for sticky parser checks.
Can the global flag break RegExp.test() checks?
Yes. A reused JavaScript RegExp with `g` or `y` can carry `lastIndex` between calls. For simple validators, remove those flags or reset `lastIndex` before each boolean check.