regex-escaped-quotes-source-parsing.md

title	Regex Trap — Escaped Quotes in Source Code Cause Premature Non-Greedy Match Termination
domain	development
tags	regex python source-parsing escape-sequences debug
status	published
source	practical-experience
confidence	0.9
created	2026-07-07
lang	en

Problem

When using regex to extract default values from Python source code like os.environ.get("KEY", "default_value"), the extraction gets truncated at escaped quotes.

Root Cause

The standard non-greedy regex (["'])(.*?)\1 fails at \" because the regex doesn't know \" is an escape sequence — it just sees a " character.

Solution

Replace . with [^\\]|\\. to skip escaped characters:

# ❌ Breaks on escaped quotes
pattern = r'(["\'])(.*?)\1'

# ✅ Handles escaped quotes
pattern = r'(['\''"'"'])((?:[^\\\\]|\\\\.)*?)\1'

Verification

import re
source = 'os.environ.get("KEY", "/@\\\\"'+:?&!=% ")'
pattern = r'(['\''"'"'])((?:[^\\\\]|\\\\.)*?)\1'
match = re.search(pattern, source)
print(match.group(2))  # /@"'+:?&!=% 

Notes

• This pattern works for any escape sequence: \", \\, \', \n, etc.
• For multi-line strings, add re.DOTALL flag

Source

Translated from Chinese lesson by zsxh1990.