js-regex

js-regex

A 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$").search("abc\n")  # matches, unlike JS
js_regex.compile("^abc$").search("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.

This only works for the .search() method - there is no equivalent to .match() or .fullmatch() for Javascript regular expressions.

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 may also raise 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	Explicit error, use ^ instead
\Z (end of string)	no	yes	Explicit error, use $ instead
(?<=text) (positive lookbehind)	new in ES2018	yes	Allowed
(?<!text) (negative lookbehind)	new in ES2018	yes	Allowed
(?(1)then|else)	no	yes	Explicit error
(?(group)then|else)	no	yes	Explicit error
(?#comment)	no	yes	Explicit error
(?P<name>regex) (Python named capture group)	no	yes	Not detected (yet)
(?P=name) (Python named backreference)	no	yes	Not detected (yet)
(?<name>regex) (JS named capture group)	new in ES2018	no	Error from Python, not translated (yet)
$<name> (JS named backreference)	new in ES2018	no	Error from Python, not translated (yet)
(?i) (case insensitive)	/i only	yes	Explicit error, compile with flags=re.IGNORECASE instead
(?m) (^ and $ match at line breaks)	/m only	yes	Explicit error, compile with flags=re.MULTILINE instead
(?s) (dot matches newlines)	no	yes	Explicit error, compile with flags=re.DOTALL instead
(?x) (free-spacing mode)	no	yes	Explicit error, there is no corresponding mode in Javascript
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

Note that in many cases Python-only regex features would be treated as part of an ordinary pattern by JS regex engines. Currently we raise an explicit error on such inputs, but may translate them to have the JS behaviour in a future version.

Changelog

1.0.1 - 2019-10-17

• Allow use of native strings on Python 2. This is not actually valid according to the spec, but it's only going to be around for a few months so whatever.

1.0.0 - 2019-10-04

• Now considered feature-complete and stable, as all constructs recommended for jsonschema patterns are supported and all Python-side incompatibilities are detected.
• Compiled patterns are now cached on Python 3, exactly as for re.compile

0.4.0 - 2019-10-03

• Now compatible with Python 2.7 and 3.5, until their respective EOL dates.

0.3.0 - 2019-09-30

• Fixed handling of non-trailing $, e.g. in "^abc$|^def$" both are converted
• Added explicit errors for re.LOCALE and re.VERBOSE flags, which have no JS equivalent
• Added explicit checks and errors for use of Python-only regex features

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.