Regex New Added 12 September 2026

Regex: test() returns false every other call with the /g flag

A regex with the g or y flag keeps lastIndex between calls, so test() resumes from where the previous match ended and alternates between true and false on the same input. It is worst when the regex is a module level constant shared by every request the process handles.

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

Quick fix

// The bug, in its usual form
const EMAIL = /\S+@\S+/g;
EMAIL.test('[email protected]');   // true
EMAIL.test('[email protected]');   // false, lastIndex is 7

// test() does not need the g flag: drop it
const EMAIL_OK = /\S+@\S+/;

// If the flag is needed elsewhere, reset before each use
EMAIL.lastIndex = 0;

// Or use the methods that do not carry state
'[email protected]'.match(EMAIL);             // with /g, all matches
[...'[email protected]'.matchAll(EMAIL)];     // requires /g, no shared cursor

// The same trap hits replace() in a loop and any shared validator object.
// Build the regex inside the function when in doubt: the cost is negligible
// next to a validator that silently passes every second request

How to diagnose Regex errors

Regex problems come in two flavours: it does not match what you expect (usually escaping, greediness, or an engine feature difference) and it matches but takes forever. The second is catastrophic backtracking, a genuine denial-of-service vector known as ReDoS, caused by nested quantifiers over overlapping character classes, such as (a+)+b. Any regex applied to untrusted input should be checked for it.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

Test against a visualiser that shows backtracking steps, and always test with input that fails to match. That is where catastrophic backtracking appears, not on successful matches.
Eliminate nested quantifiers over overlapping classes. Rewrite (a+)+ as a+, and prefer possessive quantifiers or atomic groups where the engine supports them.
Know your engine: lookbehind, named groups and Unicode property escapes differ between PCRE, RE2, JavaScript, Python and Go. RE2 (used by Go) deliberately has no backtracking and rejects some patterns outright.
Anchor patterns with ^ and $ where you mean a whole-string match. Unanchored patterns silently match substrings.
Set a timeout or use a linear-time engine for user-supplied patterns. Never run an untrusted regex on a request thread.

Tools worth reaching for

regex101.com
re2 / RE2J
Python re.DEBUG
grep -P for PCRE testing
recheck / redos linters

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

developer.mozilla.org

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Something missing or wrong?

This entry is maintained by hand. If the fix is out of date, incomplete, or you have a better one, email a correction and it will be reviewed.