regexpu-core

regexpu is a source code transpiler that enables the use of ES2015 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 ES2015 `u` flag into equivalent ES5-compatible regular expression patterns.

# Installation

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

npm install regexpu-core --save

Then, `require` it:

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

# API

This module exports a single function named `rewritePattern`.

## <tt>rewritePattern(pattern, flags, options)</tt>

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('foo.bar', '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:
rewritePattern('foo.bar');
// → 'foo(?:[\\0-\\t\\x0B\\f\\x0E-\\u2027\\u202A-\\uFFFF])bar'
// But with the ES2015 `u` flag, it matches astral symbols too:
rewritePattern('foo.bar', 'u');
// → 'foo(?:[\\0-\\t\\x0B\\f\\x0E-\\u2027\\u202A-\\uD7FF\\uDC00-\\uFFFF]|[\\uD800-\\uDBFF][\\uDC00-\\uDFFF]|[\\uD800-\\uDBFF])bar'

The optional `options` argument recognizes the following properties:

### <tt>dotAllFlag</tt> (default: <tt>false</tt>)

Setting this option to `true` enables support for the `s` (`dotAll`) flag.

rewritePattern('.');
// → '[\\0-\\t\\x0B\\f\\x0E-\\u2027\\u202A-\\uFFFF]'
rewritePattern('.', '', {
'dotAllFlag': true
});
// → '[\\0-\\t\\x0B\\f\\x0E-\\u2027\\u202A-\\uFFFF]'
rewritePattern('.', 's', {
'dotAllFlag': true
});
// → '[\\0-\\uFFFF]'
rewritePattern('.', 'su', {
'dotAllFlag': true
});
// → '(?:[\\0-\\uD7FF\\uE000-\\uFFFF]|[\\uD800-\\uDBFF][\\uDC00-\\uDFFF]|[\\uD800-\\uDBFF](?![\\uDC00-\\uDFFF])|(?:[^\\uD800-\\uDBFF]|^)[\\uDC00-\\uDFFF])'

### <tt>unicodePropertyEscape</tt> (default: <tt>false</tt>)

Setting this option to `true` enables support for Unicode property escapes:

rewritePattern('\\p{Script_Extensions=Anatolian_Hieroglyphs}', 'u', {
'unicodePropertyEscape': true
});
// → '(?:\\uD811[\\uDC00-\\uDE46])'

### <tt>lookbehind</tt> (default: <tt>false</tt>)

Setting this option to `true` enables support for lookbehind assertions.

rewritePattern('(?<=.)a', '', {
'lookbehind': true
});
// → '(?<=[\\0-\\t\\x0B\\f\\x0E-\\u2027\\u202A-\\uFFFF])a'

### <tt>namedGroup</tt> (default: <tt>false</tt>)

Setting this option to `true` enables support for named capture groups.

rewritePattern('(?<name>.)\k<name>', '', {
'namedGroup': true
});
// → '(.)\1'

### <tt>onNamedGroup</tt>

This option is a function that gets called when a named capture group is found. It receives two parameters: the name of the group, and its index.

rewritePattern('(?<name>.)\k<name>', '', {
'namedGroup': true,
onNamedGroup(name, index) {
console.log(name, index);
// → 'name', 1
}
});

### <tt>useUnicodeFlag</tt> (default: <tt>false</tt>)

Setting this option to `true` enables the use of Unicode code point escapes of the form `\u{…}`. Note that in regular expressions, such escape sequences only work correctly when the ES2015 `u` flag is set. Enabling this setting often results in more compact output, although there are cases (such as `\p{Lu}`) where it actually increases the output size.

rewritePattern('\\p{Script_Extensions=Anatolian_Hieroglyphs}', 'u', {
'unicodePropertyEscape': true,
'useUnicodeFlag': true
});
// → '[\\u{14400}-\\u{14646}]'

# Author

Mathias Bynens

# License

regexpu-core is available under the MIT license.