Regex New Added 19 September 2026

Java: PatternSyntaxException: Illegal repetition

A brace appears where the engine expects a quantifier, so a literal { in the pattern is read as the start of {n,m} and rejected. It shows up most often when a template placeholder such as {name} is passed straight to Pattern.compile or to String.replaceAll.

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

// Fails: {name} looks like a malformed quantifier
Pattern.compile("Hello {name}");

// Escape the brace, or the whole literal
Pattern.compile("Hello \\{name\\}");
Pattern.compile(Pattern.quote("Hello {name}"));

// replaceAll and split take a regex, replace does not
"Hello {name}".replace("{name}", user);              // no regex involved
"Hello {name}".replaceAll(Pattern.quote("{name}"), Matcher.quoteReplacement(user));

// The replacement string has its own syntax: $ and backslash are special
Matcher.quoteReplacement("cost: $5");

// Remember Java string escaping happens first, so a regex backslash is doubled
Pattern.compile("\\d{2,4}");    // the regex the engine sees is \d{2,4}

// Build patterns from user input with quote(), never by concatenation

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.

docs.oracle.com docs.oracle.com

Related Regex errors

See all 11 Regex errors →

Browse other categories

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.