Python

re.error: What It Means and How to Fix It

Error reference

re.error

Language

Python

Severity

Medium

When it happens

re.error is raised when a string passed to a function in the re module is not a valid regular expression.

Potential fixes

Test regex patterns interactively with a tool like regex101.com before embedding them in code. Always wrap re.compile() in a try/except re.error block when the pattern comes from user input. Use raw strings (r'...') to avoid Python string escape conflicts.

Related errors

ValueError SyntaxError

Deep dive

re.error is raised when a string passed to a function in the re module is not a valid regular expression. The error message includes the position of the syntax problem in the pattern, making it straightforward to locate and fix.

Why it happens

•Unclosed parentheses or brackets in the pattern.
•An invalid escape sequence like \p or \q that is not a recognized regex token.
•A quantifier like * or + applied to nothing (at the start of the pattern).
•Unmatched curly braces in a repetition quantifier like {3,.

Potential fixes

Test regex patterns interactively with a tool like regex101.com before embedding them in code. Always wrap re.compile() in a try/except re.error block when the pattern comes from user input. Use raw strings (r'...') to avoid Python string escape conflicts.

Examples

Python

Code that triggers the error

import re
re.compile(r"(unclosed group")

Error output

re.error: missing ), unterminated subpattern at position 0

Fixed code

import re

try:
    pattern = re.compile(r"(closed group)")
    match = pattern.search("closed group found")
    print(match.group() if match else "no match")
except re.error as e:
    print(f"Invalid regex: {e}")

Practice in English

How would you explain a re.error to a fellow dev? Choose the right phrase:

"I hit an error...

Ready to practice your English at work?

Lingua-e has interactive exercises built around real developer conversations: standups, code reviews, retrospectives, and more. Practice until it comes naturally.

Try Lingua-e for free