Regex New Added 29 September 2026

Python: re.error: global flags not at the start of the expression

An inline flag such as (?i) or (?s) appears somewhere other than the very start of the pattern. Python 3.6 to 3.10 only warned about it, and 3.11 made it an error, so code that has worked for years breaks on upgrade. It is common when patterns are built by joining strings together, each with its own flag prefix.

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

import re

# Before: flag in the middle, an error since Python 3.11
# re.compile(r'user: (?i)admin')

# Fix 1: move the flag to the start
re.compile(r'(?i)user: admin')

# Fix 2: pass it as an argument
re.compile(r'user: admin', re.IGNORECASE)

# Fix 3: scope it to just one group
re.compile(r'user: (?i:admin)')

# Joined patterns: strip the prefixes and apply the flag once
parts = [r'foo', r'bar']
re.compile('|'.join(parts), re.IGNORECASE)

# Find offenders before upgrading
# python3.10 -W error::DeprecationWarning -m pytest

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 docs.python.org

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