Regex New Added 10 September 2026

Regex: nothing to repeat at position 0

A quantifier appeared with nothing before it to repeat, so the pattern is invalid rather than merely wrong. It is almost always a pattern built at runtime: user input interpolated into a regex, or a string that already contained a plus or asterisk meant literally.

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

# Invalid: the quantifier has no target
re.compile("+44")        # nothing to repeat at position 0
re.compile("a**")        # multiple repeat

# Escape it, or quote the whole interpolated fragment
re.compile(r"\+44")
re.compile(re.escape(user_input))

// JavaScript has the same rule and a similar message
new RegExp("\\+44");            // from a string, both slashes needed
new RegExp(input.replace(/[.*+?^\${}()|[\]\\]/g, "\\$&"));

# Never build a pattern from input without escaping: it is a correctness bug
# before it is a security one

# Test the pattern in isolation before it reaches production
python3 -c "import re; re.compile(open('pattern.txt').read().strip())"

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.python.org

Related Regex errors

See all 8 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.