re.error: Unterminated Subpattern

Why This Happens

Parentheses in regex define groups. If not balanced, the engine raises an error. Escape literal parentheses with \( and \).

The Problem

import re
result = re.match(r'(hello', 'hello')

The Fix

import re
result = re.match(r'(hello)', 'hello')
# Literal parens: r'\(hello\)'

Step-by-Step Fix

1. 1

   Balance parentheses

   Count opening and closing parentheses.

2. 2

   Escape literal characters

   Use backslash for literal parens, brackets.

3. 3

   Use re.escape()

   Escape all special characters in a user-provided string.