CodeQL documentation

Unmatchable dollar in regular expression

ID: py/regex/unmatchable-dollar
Kind: problem
Severity: error
Precision: high
Tags:
   - reliability
   - correctness
Query suites:
   - python-security-and-quality.qls

Click to see the query in the CodeQL repository

A dollar assertion $ in a regular expression only matches at the end of the input, or (for multi-line regular expressions) at the end of a line. If it is followed by a pattern that must match a non-empty sequence of (non-newline) input characters, it cannot possibly match, rendering the entire regular expression unmatchable.

Recommendation

Examine the regular expression to find and correct any typos.

Example

In the following example, the regular expression r"\.\(\w+$\)" cannot match any string, since it contains a dollar assertion followed by an escape sequence that matches a closing parenthesis.

The second regular expression, r"\.\(\w+\)$", has the dollar at the end and will work as expected.

import re
#Regular expression that includes a dollar, but not at the end.
matcher = re.compile(r"\.\(\w+$\)")

def find_it(filename):
    if matcher.match(filename):
        print("Found it!")

#Regular expression anchored to end of input.
fixed_matcher = re.compile(r"\.\(\w+\)$")

References