EC
re.error
PythonERRORNotableValue ErrorHIGH confidence

Invalid regular expression

Production Risk

Always validate user-supplied regex patterns; catastrophic backtracking is a separate DoS risk.

What this means

Raised by the re module when a regular expression pattern is invalid — for example, mismatched parentheses, invalid escape sequences, or quantifiers in invalid positions.

Why it happens
  1. 1Unmatched parentheses: re.compile("(foo")
  2. 2Invalid escape sequence: re.compile("\p") in Python (not supported)
  3. 3Quantifier without atom: re.compile("*foo")
  4. 4Lookbehind with variable width: re.compile("(?<=a+)b")
How to reproduce

Compiling a regex with an unmatched parenthesis.

trigger — this will error
trigger — this will error
import re
re.compile("(unclosed group")

expected output

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

Fix 1

Validate regex patterns from user input

WHEN Accepting regex patterns from users or configuration

Validate regex patterns from user input
import re

def safe_compile(pattern):
    try:
        return re.compile(pattern)
    except re.error as e:
        raise ValueError(f"Invalid regex pattern {pattern!r}: {e}") from e

Why this works

Wrap re.compile() in try/except re.error to give users a clear error message.

Fix 2

Use raw strings for regex patterns

WHEN Writing regex literals in Python

Use raw strings for regex patterns
import re
# Use raw strings (r"") to avoid double-escaping
pattern = re.compile(r"\d+\.\d+")  # matches 3.14
# Without r"", you'd need "\\d+\\.\\d+"

Why this works

Raw strings prevent Python from interpreting backslashes before the regex engine sees them.

Code examples
Triggerpython
import re
re.compile("(unclosed group")  # re.error: missing )
Handle with try/exceptpython
import re
try:
    pattern = re.compile(user_regex)
except re.error as e:
    print(f"Invalid regex: {e}")
    pattern = re.compile("")
Avoid with pre-validationpython
import re
def safe_compile(pat):
    try:
        return re.compile(pat)
    except re.error as e:
        raise ValueError(f"Invalid pattern {pat!r}: {e}") from e
Sources
Official documentation ↗

Python Docs — re module

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