Regex New Added 29 September 2026

Pattern attribute value [a-z-_]+ is not a valid regular expression

Browsers now compile the HTML pattern attribute with the v (unicodeSets) flag, which is stricter about character classes than plain JavaScript regex. A hyphen between other characters, and unescaped characters such as ( ) [ ] { } / | inside a class, are now syntax errors. The console shows the warning and the browser silently skips validation, so the form accepts anything.

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

<!-- Before: worked for years, now invalid under the v flag -->
<input pattern="[a-z-_]+">
<input pattern="[\w.-/]+">

<!-- After: put the hyphen last, escape special characters -->
<input pattern="[a-z_\-]+">
<input pattern="[\w.\/\-]+">

// Test a pattern the way the browser does
new RegExp('^(?:' + '[a-z-_]+' + ')$', 'v');  // throws: invalid pattern
new RegExp('^(?:' + '[a-z_\\-]+' + ')$', 'v');  // fine

// Check the console for 'is not a valid regular expression' on every form page

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.

html.spec.whatwg.org developer.mozilla.org

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