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regexpu is a source code transpiler that enables the use of ES6 Unicode regular expressions in JavaScript-of-today (ES5).

regexpu-core contains regexpu’s core functionality, i.e. rewritePattern(pattern, flag), which enables rewriting regular expressions that make use of the ES6 u flag into equivalent ES5-compatible regular expression patterns.


To use regexpu-core programmatically, install it as a dependency via npm:

npm install regexpu-core --save-dev

Then, require it:

const rewritePattern = require('regexpu-core');


This module exports a single function named rewritePattern.

rewritePattern(pattern, flags)

This function takes a string that represents a regular expression pattern as well as a string representing its flags, and returns an ES5-compatible version of the pattern.

rewritePattern('', 'u');
// → 'foo(?:[\\0-\\t\\x0B\\f\\x0E-\\u2027\\u202A-\\uD7FF\\uDC00-\\uFFFF]|[\\uD800-\\uDBFF][\\uDC00-\\uDFFF]|[\\uD800-\\uDBFF])bar'

rewritePattern('[\\u{1D306}-\\u{1D308}a-z]', 'u');
// → '(?:[a-z]|\\uD834[\\uDF06-\\uDF08])'

rewritePattern('[\\u{1D306}-\\u{1D308}a-z]', 'ui');
// → '(?:[a-z\\u017F\\u212A]|\\uD834[\\uDF06-\\uDF08])'

regexpu-core can rewrite non-ES6 regular expressions too, which is useful to demonstrate how their behavior changes once the u and i flags are added:

// In ES5, the dot operator only matches BMP symbols:
// → 'foo(?:[\\0-\\t\\x0B\\f\\x0E-\\u2027\\u202A-\\uFFFF])bar'

// But with the ES6 `u` flag, it matches astral symbols too:
rewritePattern('', 'u');
// → 'foo(?:[\\0-\\t\\x0B\\f\\x0E-\\u2027\\u202A-\\uD7FF\\uDC00-\\uFFFF]|[\\uD800-\\uDBFF][\\uDC00-\\uDFFF]|[\\uD800-\\uDBFF])bar'

rewritePattern uses regjsgen, regjsparser, and regenerate as internal dependencies.


Mathias Bynens


regexpu-core is available under the MIT license.