js-regex 0.2.0

A thin compatibility layer to use Javascript regular expressions in Python

js-regex

A thin compatibility layer to use Javascript regular expressions in Python.

Did you know that regular expressions may vary between programming languages? For example, let's consider the pattern "^abc$", which matches the string "abc". But what about the string "abc\n"? It's also matched in Python, but not in Javascript!

This and other slight differences can be really important for cross-language standards like jsonschema, and that's why js-regex exists.

How it works

import re
import js_regex

re.compile("^abc$").match("abc\n")  # matches, unlike JS
js_regex.compile("^abc$").match("abc\n")  # does not match

Internally, js_regex.compile() replaces JS regex syntax which has a different meaning in Python with whatever Python regex syntax has the intended meaning.

We also check for constructs which are valid in Python but not JS - such as named capture groups - and raise an explicit error. Constructs which are valid in JS but not Python are also an error, because we're still using Python's re.compile() function under the hood!

The following table is adapted from this larger version, ommiting other languages and any rows where JS and Python have the same behaviour.

Feature	Javascript	Python	Handling
\a (bell)	no	yes	Converted to JS behaviour
\ca-\cz and \cA-\cZ (control characters)	yes	no	Converted to JS behaviour
\d for digits, \w for word chars, \s for whitespace	ascii	unicode	Converted to JS behaviour (including \D, \W, \S for negated classes)
$ (end of line/string)	at end	allows trailing \n	Converted to JS behaviour
\A (start of string)	no	yes	TODO: explicit error, suggest ^
\Z (end of string)	no	yes	TODO: explicit error, suggest $
(?<=text) (positive lookbehind)	no	yes	TODO: explicit error
(?<!text) (negative lookbehind)	no	yes	TODO: explicit error
(?(1)then|else)	no	yes	TODO: explicit error
(?(group)then|else)	no	yes	TODO: explicit error
(?#comment)	no	yes	TODO: explicit error
(?P<name>regex) (named capture group)	no	yes	TODO: explicit error
(?P=name) (named backreference)	no	yes	TODO: explicit error
Free-spacing / multi-line syntax	no	yes	TODO: explicit error
(?s) (dot matches newlines)	no	yes	TODO: explicit error
(?x) (free-spacing mode)	no	yes	TODO: explicit error
(?i) (case insensitive)	/i only	yes	TODO: ???
(?m) (^ and $ match at line breaks)	/m only	yes	TODO: ???
Backreferences non-existent groups are an error	no	yes	Follows Python behaviour
Backreferences to failed groups also fail	no	yes	Follows Python behaviour
Nested references \1 through \9	yes	no	Follows Python behaviour

Changelog

0.2.0 - 2019-09-28

Convert JS-only syntax to Python equivalent wherever possible.

0.1.0 - 2019-09-28

Initial release, with project setup and a very basic implementation.