Python

Describes how recheck checks a regular expression of the python dialect.

Engine

recheck follows the syntax and the semantics of Python 3.15's re module.

Settings

• type is the string type of the pattern and its input, str (default) or bytes.

CLI

$ recheck --dialect=python '/^(a|a)*$/'
$ recheck --dialect=python --python-type=bytes '/^(a|a)*$/'

JavaScript

import { check } from "recheck";

await check("^(a|a)*$", "", { dialect: "python" });
await check("^(a|a)*$", "", { dialect: { name: "python", type: "bytes" } });

Scala

import codes.quine.labs.recheck.ReDoS
import codes.quine.labs.recheck.common.Parameters
import codes.quine.labs.resyntax.dialect.Dialect

ReDoS.check("^(a|a)*$", "", Parameters(dialect = Dialect.Python()))
ReDoS.check("^(a|a)*$", "", Parameters(dialect = Dialect.Python(Dialect.Python.Type.Bytes)))

Flags and Units

The matching unit is code-point for a str pattern (the default type), and byte for a bytes pattern (see Matching Units).

recheck supports the following re flags, written as their letters.

Flag	re flag	Inline	Notes
a	re.ASCII (re.A)	(?a)	An error with u or L.
i	re.IGNORECASE (re.I)	(?i)
L	re.LOCALE (re.L)	(?L)	Only in a bytes pattern.
m	re.MULTILINE (re.M)	(?m)
s	re.DOTALL (re.S)	(?s)
u	re.UNICODE (re.U)	(?u)	The default of a str pattern, and an error in a bytes pattern.
x	re.VERBOSE (re.X)	(?x)

Syntax Limitations

\N{...} (e.g. \N{EM DASH}) is reported as unsupported, as in the other dialects having it.

Approximations

• L follows the C locale, where 0x80–0xFF follow the ASCII rules, whatever locale the environment has.
• A conditional on an open group (e.g. (?(1)b|c) in group 1) counts the group as unset when the end of its previous match is before the position where it opens again, and recheck does not reproduce it. For example, ^(?:(a(?(1)b|c))x)+$ matches "acxacx" in Python 3.15, but recheck matches "acxabx" as the other engines do.

Runtime-Attack Verification

• Command: python3 -I -.
• Matching: re.search.
• Engine limits: none.
• Engine errors: RecursionError, MemoryError.
• Environment variable: RECHECK_PYTHON.