Regex recompiled in loop (ARCH2073)

Key python.performance.regex-recompiled-in-loop · Severity info · Category performance

Compiling a constant pattern on every loop iteration is wasteful. Compile the pattern once with re.compile outside the loop and reuse it.

Incorrect

import re

def find_codes(lines):
    out = []
    for line in lines:
        if re.match(r"^[A-Z]{3}\d{4}$", line):
            out.append(line)
    return out

Correct

import re

CODE_RE = re.compile(r"^[A-Z]{3}\d{4}$")

def find_codes(lines):
    return [line for line in lines if CODE_RE.match(line)]

Fix

Compile the constant pattern once at module level with re.compile outside the loop.