oniguruma-to-es

Oniguruma (鬼車) to ES

“I think [Oniguruma-To-ES] is very wonderful”
— K. Kosako, creator of Oniguruma

Oniguruma is a regular expression engine written in C that's used in Ruby (via a fork named Onigmo), PHP (mb_ereg, etc.), TextMate grammars (used by VS Code, Shiki, etc. for syntax highlighting), and many other tools.

Oniguruma-To-ES is an advanced Oniguruma to JavaScript regex translator that runs in the browser or the server, with support for ~99.99% of Oniguruma regexes (more details below). Use it to:

• Take advantage of Oniguruma's many extended regex features in JavaScript.
• Run regexes written for Oniguruma from JavaScript.
• Share regexes across your Ruby or PHP and JavaScript code.

Compared to running the Oniguruma C library via WASM using vscode-oniguruma, this library is ~4% of the size and its regexes often run much faster (even including transpilation time) since they run as native JavaScript.

[!TIP] You can further reduce bundle size (and increase run-time performance) by precompiling your regexes. In many cases, that avoids the need for any run-time dependency. Conversions for regexes that use certain advanced features rely on a RegExp subclass, in which case the tree-shakable EmulatedRegExp (3 kB minzip) is still needed after precompilation.

Oniguruma-To-ES deeply understands the hundreds of large and small differences between Oniguruma and JavaScript regex syntax and behavior, across multiple JavaScript version targets. It's obsessive about ensuring that the emulated features it supports have exactly the same behavior, even in extreme edge cases. And it's been battle-tested on tens of thousands of real-world Oniguruma regexes used in TextMate grammars. It's built on top of oniguruma-parser and Regex+, both by the same author as this library.

🧪 Try the demo REPL

📜 Contents

• Examples
• Install and use
• API: toRegExp, toRegExpDetails, EmulatedRegExp
• Options: accuracy, avoidSubclass, flags, global, hasIndices, lazyCompileLength, rules, target, verbose
• Supported features
• Unsupported features
• Unicode

🪧 Examples

import {toRegExp} from 'oniguruma-to-es';

toRegExp(String.raw`(?x)
  (?<n>\d) (?<n>\p{greek}) \k<n>
  ([0a-z&&\h]){,2}
`);
// → /(?<n>\p{Nd})(\p{sc=Greek})(?>\2|\1)(?:[[0a-z]&&\p{AHex}]){0,2}/v

Although the example above is fairly straightforward, you can see several translations that might not be obvious. Apart from the (?x) free-spacing modifier and the \h hex-digit shorthand that aren't available in JavaScript, you can also see that Oniguruma's \d is Unicode-based by default, backreferences to duplicate group names match the captured value of any of the groups, (…) groups are noncapturing by default if named groups are present, character class intersection doesn't follow JavaScript's requirement of using nested classes for union and ranges, and {…} interval quantifiers can use an implicit 0 min. Many advanced features are supported that would produce more complicated transformations.

If you have a keen eye, you might have noticed that the result used an atomic group (?>…), which JavaScript doesn't natively support. That was a simplification for readability; the actual result uses (?=(\2|\1))\3 to accomplish the same effect, and then uses a RegExp subclass to automatically remove the added capturing group from reported match results.

This next example shows support for Unicode case folding with mixed case-sensitivity. Notice that code points ſ (U+017F) and K (U+212A) are added to the second, case-insensitive range if using a target prior to ES2025, and that modern JavaScript regex features (like flag groups) are used if supported by the target.

toRegExp('[a-z](?i)[a-z]', {target: 'ES2018'});
// → /[a-z][a-zA-ZſK]/u
toRegExp('[a-z](?i)[a-z]', {target: 'ES2025'});
// → /[a-z](?i:[a-z])/v

🕹️ Install and use

npm install oniguruma-to-es

import {toRegExp} from 'oniguruma-to-es';

const str = '…';
const pattern = '…';
// Works with all string/regexp methods since it returns a native regexp
str.match(toRegExp(pattern));

Using a global name (no import)

<script src="https://cdn.jsdelivr.net/npm/oniguruma-to-es/dist/index.min.js"></script>
<script>
  const {toRegExp} = OnigurumaToEs;
</script>

🔑 API

toRegExp

Accepts an Oniguruma pattern and returns an equivalent JavaScript RegExp.

[!TIP] Try it in the demo REPL.

function toRegExp(
  pattern: string,
  options?: ToRegExpOptions
): RegExp | EmulatedRegExp;

Type ToRegExpOptions

type ToRegExpOptions = {
  accuracy?: 'default' | 'strict';
  avoidSubclass?: boolean;
  flags?: string;
  global?: boolean;
  hasIndices?: boolean;
  lazyCompileLength?: number;
  rules?: {
    allowOrphanBackrefs?: boolean;
    asciiWordBoundaries?: boolean;
    captureGroup?: boolean;
    recursionLimit?: number;
    singleline?: boolean;
  };
  target?: 'auto' | 'ES2025' | 'ES2024' | 'ES2018';
  verbose?: boolean;
};

See Options for more details.

toRegExpDetails

Accepts an Oniguruma pattern and returns the details needed to construct an equivalent JavaScript RegExp.

function toRegExpDetails(
  pattern: string,
  options?: ToRegExpOptions
): {
  pattern: string;
  flags: string;
  options?: EmulatedRegExpOptions;
};

Note that the returned flags might also be different than those provided, as a result of the emulation process. The returned pattern, flags, and options properties can be provided as arguments to the EmulatedRegExp constructor to produce the same result as toRegExp.

If the only keys returned are pattern and flags, they can optionally be provided to JavaScript's RegExp constructor instead. Setting option avoidSubclass to true ensures that this is always the case (resulting in an error for any patterns that require EmulatedRegExp's additional handling).

EmulatedRegExp

Works the same as JavaScript's native RegExp constructor in all contexts, but can be given results from toRegExpDetails to produce the same result as toRegExp.

class EmulatedRegExp extends RegExp {
  constructor(pattern: string, flags?: string, options?: EmulatedRegExpOptions);
  constructor(pattern: EmulatedRegExp, flags?: string);
  rawOptions: EmulatedRegExpOptions;
}

The rawOptions property of EmulatedRegExp instances can be used for serialization.

Type EmulatedRegExpOptions

type EmulatedRegExpOptions = {
  hiddenCaptures?: Array<number>;
  lazyCompile?: boolean;
  strategy?: string | null;
  transfers?: Array<[number, Array<number>]>;
};

🔩 Options

The following options are shared by functions toRegExp and toRegExpDetails.

accuracy

One of 'default' (default) or 'strict'.

Sets the level of emulation rigor/strictness.

• Default: Permits a few close approximations in order to support additional features.
• Strict: Error if the pattern can't be emulated with identical behavior (even in rare edge cases) for the given target.

More details

Using default accuracy adds support for the following features, depending on target:

• All targets (ES2025 and earlier):
  • Enables use of \X using a close approximation of a Unicode extended grapheme cluster.
  • Enables combining lookbehind with uncommon uses of \G that rely on subclass-based emulation.
• ES2024 and earlier:
  • Enables use of case-insensitive backreferences to case-sensitive groups.
• ES2018:
  • Enables use of POSIX classes [:graph:] and [:print:] using ASCII-based versions rather than the Unicode versions available for ES2024 and later. Other POSIX classes are always Unicode-based.

avoidSubclass

Default: false.

Disables advanced emulation that relies on returning a RegExp subclass. In cases when a subclass would otherwise have been used, this results in one of the following:

• An error is thrown for patterns that are not emulatable without a subclass.
• Some patterns can still be emulated accurately without a subclass, but in this case subpattern match details might differ from Oniguruma.
  • This is only relevant if you access the subpattern details of match results in your code (via subpattern array indices, groups, and indices).

flags

Oniguruma flags; a string with i, m, x, D, S, W in any order (all optional).

Flags can also be specified via modifiers in the pattern.

[!IMPORTANT] Oniguruma and JavaScript both have an m flag but with different meanings. Oniguruma's m is equivalent to JavaScript's s (dotAll).

global

Default: false.

Include JavaScript flag g (global) in the result.

hasIndices

Default: false.

Include JavaScript flag d (hasIndices) in the result.

lazyCompileLength

Default: Infinity. In other words, lazy compilation is off by default.

Delay regex construction until first use if the transpiled pattern is at least this length.

Although regex construction in JavaScript is fast, it can sometimes be helpful to defer the cost for extremely long patterns. Lazy compilation defers the time JavaScript spends inside the RegExp constructor (building the transpiled pattern into a regex object) until the first time the regex is used in a search. The regex object is outwardly identical before and after deferred compilation.

Lazy compilation relies on the EmulatedRegExp class.

rules

Advanced options that override standard behavior, error checking, and flags when enabled.

• allowOrphanBackrefs: Useful with TextMate grammars that merge backreferences across patterns.
• asciiWordBoundaries: Use ASCII-based \b and \B, which increases search performance of generated regexes.
• captureGroup: Allow unnamed captures and numbered calls (backreferences and subroutines) when using named capture.
  • This is Oniguruma option ONIG_OPTION_CAPTURE_GROUP; on by default in vscode-oniguruma.
• recursionLimit: Change the recursion depth limit from Oniguruma's 20 to an integer 2–20.
• singleline: ^ as \A; $ as \Z. Improves search performance of generated regexes without changing the meaning if searching line by line.
  • This is Oniguruma option ONIG_OPTION_SINGLELINE.

target

One of 'auto' (default), 'ES2025', 'ES2024', or 'ES2018'.

JavaScript version used for generated regexes. Using auto detects the best value for your environment. Later targets enable faster transpilation, simpler generated source, and support for additional features.

More details

• ES2018: Uses JS flag u.
  • Emulation restrictions: Character class intersection and some uses of nested, negated character classes aren't supported.
  • Generated regexes might use ES2018 features that require Node.js 10 or a browser version released during 2018 to 2023 (in Safari's case). Minimum requirement for any regex is Node.js 6 or a 2016-era browser.
• ES2024: Uses JS flag v.
  • No emulation restrictions.
  • Generated regexes require Node.js 20 or any 2023-era browser (compat table).
• ES2025: Uses JS flag v and allows use of flag groups.
  • Benefits: Faster transpilation, simpler generated source.
  • Generated regexes might use features that require Node.js 23 or a 2024-era browser (except Safari, which lacks support for flag groups).

verbose

Default: false.

Disables minifications that simplify the pattern without changing the meaning.

Example: By default, unneeded noncapturing groups might be removed during transpilation. Setting this option to true disables such changes.

[!TIP] The oniguruma-parser library includes a regex optimizer that goes far beyond the basic, built-in minifications. If desired, you can call the optimizer first, and then use its result for transpilation. That isn't appropropriate in all cases (since it adds performance overhead and increases bundle size), but the benefits of optimization do pass through to the transpiled, JavaScript version of a regex.

✅ Supported features

Following are the supported features by target. The official Oniguruma syntax doc doesn't cover many of the finer details described here.

[!NOTE] Targets ES2024 and ES2025 have the same emulation capabilities. Resulting regexes might have different source and flags, but they match the same strings. See target.

🆕 = Syntax not available in JavaScript.
🆚 = JavaScript uses slightly different syntax for the same concept; ex: \x{…} → \u{…}.

Even for features not marked with one of the above symbols, notice that nearly every feature below has at least subtle differences from JavaScript. Unsupported features throw an error.

Feature		Example	ES2018	ES2024+	Subfeatures & JS differences
Characters	Literal	E, !	✅	✅	✔ Code point based matching (same as JS with flag u, v) ✔ Standalone ], {, } don't require escaping
	Identity escape	\E, \!	✅	✅	✔ Different set than JS ✔ Allows multibyte chars
	Escaped metachar	\\, \.	✅	✅	✔ Same as JS
	Control code escape	\t	✅	✅	✔ The JS set plus \a, \e
	\xNN	\x7F	✅	✅	✔ Allows 1 hex digit ✔ Above 7F, is UTF-8 encoded byte (≠ JS) ✔ Error for invalid encoded bytes
	\uNNNN	\uFFFF	✅	✅	✔ Same as JS with flag u, v
	🆚 \x{…}	\x{A}	✅	✅	✔ Allows leading 0s up to 8 total hex digits
	Escaped num	\20	✅	✅	✔ Can be backref, error, null, octal, identity escape, or any of these combined with literal digits, based on complex rules that differ from JS ✔ Always handles escaped single digit 1-9 outside char class as backref ✔ Allows null with 1-3 0s ✔ Error for octal ≥ 200
	Caret notation	\cA, 🆚 \C-A	✅	✅	✔ With A-Za-z (JS: only \c form)
Character sets	Digit	\d, \D	✅	✅	✔ Unicode by default (≠ JS)
	Word	\w, \W	✅	✅	✔ Unicode by default (≠ JS)
	Whitespace	\s, \S	✅	✅	✔ Unicode by default ✔ No JS adjustments to Unicode set (−\uFEFF, +\x85)
	🆕 Hex digit	\h, \H	✅	✅	✔ ASCII
	Dot	.	✅	✅	✔ Excludes only \n (≠ JS)
	🆕 Any	\O	✅	✅	✔ Any char (with any flags) ✔ Identity escape in char class
	🆕 Not \n	\N	✅	✅	✔ Identity escape in char class
	🆕 Newline	\R	✅	✅	✔ Matched atomically ✔ Identity escape in char class
	🆕 Grapheme	\X	☑️	☑️	● Uses a close approximation ✔ Matched atomically ✔ Identity escape in char class
	Unicode property	\p{L}, \P{L}	✅	✅	✔ Binary properties ✔ Categories ✔ Scripts ✔ Aliases ✔ POSIX properties ✔ Invert with \p{^…}, \P{^…} ✔ Insignificant spaces, hyphens, underscores, and casing in names ✔ \p, \P without { is an identity escape ✔ Error for key prefixes ✔ Error for props of strings ❌ Blocks (wontfix[1])
Character classes	Base	[…], [^…]	✅	✅	✔ Unescaped - outside of range is literal in some contexts (different than JS rules in any mode) ✔ Leading unescaped ] is literal ✔ Fewer chars require escaping than JS
	Range	[a-z]	✅	✅	✔ Same as JS with flag u, v ✔ Allows \x{…} above 10FFFF at end of range to mean last valid code point
	🆕 POSIX class	[[:word:]], [[:^word:]]	☑️[2]	✅	✔ All use Unicode definitions
	Nested class	[…[…]]	☑️[3]	✅	✔ Same as JS with flag v
	Intersection	[…&&…]	❌	✅	✔ Doesn't require nested classes for intersection of union and ranges ✔ Allows empty segments
Assertions	Line start, end	^, $	✅	✅	✔ Always "multiline" ✔ Only \n as newline ✔ ^ doesn't match after string-terminating \n
	🆕 String start, end	\A, \z	✅	✅	✔ Same as JS ^ $ without JS flag m
	🆕 String end or before terminating newline	\Z	✅	✅	✔ Only \n as newline
	🆕 Search start	\G	✅	✅	✔ Matches at start of match attempt (not end of prev match; advances after 0-length match)
	Word boundary	\b, \B	✅	✅	✔ Unicode based (≠ JS)
	Lookaround	(?=…), (?!…), (?<=…), (?<!…)	✅	✅	✔ Allows variable-length quantifiers and alternation within lookbehind ✔ Lookahead invalid within lookbehind ✔ Capturing groups invalid within negative lookbehind ✔ Negative lookbehind invalid within positive lookbehind
Quantifiers	Greedy, lazy	*, +?, {2,}, etc.	✅	✅	✔ Includes all JS forms ✔ Adds {,n} for min 0 ✔ Explicit bounds have upper limit of 100,000 (unlimited in JS) ✔ Error with assertions (same as JS with flag u, v) and directives
	🆕 Possessive	?+, *+, ++, {3,2}	✅	✅	✔ + suffix doesn't make {…} quantifiers possessive (creates a quantifier chain) ✔ Reversed {…} ranges are possessive
	🆕 Chained	**, ??+*, {2,3}+, etc.	✅	✅	✔ Further repeats the preceding repetition
Groups	Noncapturing	(?:…)	✅	✅	✔ Same as JS
	🆕 Atomic	(?>…)	✅	✅	✔ Supported
	Capturing	(…)	✅	✅	✔ Is noncapturing if named capture present
	Named capturing	(?<a>…), 🆚 (?'a'…)	✅	✅	✔ Duplicate names allowed (including within the same alternation path) unless directly referenced by a subroutine ✔ Error for names invalid in Oniguruma (more permissive than JS)
Backreferences	Numbered	\1	✅	✅	✔ Error if named capture used ✔ Refs the most recent of a capture/subroutine set
	🆕 Enclosed numbered, relative	\k<1>, \k'1', \k<-1>, \k'-1'	✅	✅	✔ Error if named capture used ✔ Allows leading 0s ✔ Refs the most recent of a capture/subroutine set ✔ \k without < or ' is an identity escape
	Named	\k<a>, 🆚 \k'a'	✅	✅	✔ For duplicate group names, rematch any of their matches (multiplex), atomically ✔ Refs the most recent of a capture/subroutine set (no multiplex) ✔ Combination of multiplex and most recent of capture/subroutine set if duplicate name is indirectly created by a subroutine ✔ Error for backref to valid group name that includes -/+
	To nonparticipating groups		☑️	☑️	✔ Error if group to the right[4] ✔ Duplicate names (and subroutines) to the right not included in multiplex ✔ Fail to match (or don't include in multiplex) ancestor groups and groups in preceding alternation paths ❌ Some rare cases are indeterminable at compile time and use the JS behavior of matching an empty string
Subroutines	🆕 Numbered, relative	\g<1>, \g'1', \g<-1>, \g'-1', \g<+1>, \g'+1'	✅	✅	✔ Error if named capture used ✔ Allows leading 0s All subroutines (incl. named): ✔ Allowed before reffed group ✔ Can be nested (any depth) ✔ Reuses flags from the reffed group (ignores local flags) ✔ Replaces most recent captured values (for backrefs) ✔ \g without < or ' is an identity escape
	🆕 Named	\g<a>, \g'a'	✅	✅	● Same behavior as numbered ✔ Error if reffed group uses duplicate name
Recursion	🆕 Full pattern	\g<0>, \g'0'	☑️[5]	☑️[5]	✔ 20-level depth limit
	🆕 Numbered, relative, named	(…\g<1>?…), (…\g<-1>?…), (?<a>…\g<a>?…), etc.	☑️[5]	☑️[5]	✔ 20-level depth limit
Other	Alternation	…|…	✅	✅	✔ Same as JS
	🆕 Absence repeater[6]	(?~…)	✅	✅	✔ Supported
	🆕 Comment group	(?#…)	✅	✅	✔ Allows escaping \), \\ ✔ Comments allowed between a token and its quantifier ✔ Comments between a quantifier and the ?/+ that makes it lazy/possessive changes it to a quantifier chain
	🆕 Fail[7]	(*FAIL)	✅	✅	✔ Supported
	🆕 Keep	\K	☑️	☑️	● Supported at top level if no top-level alternation is used
	JS features unknown to Oniguruma are handled using Oniguruma syntax rules		✅	✅	✔ \u{…} is an error ✔ [], [^] are errors ✔ [\q{…}] matches q, etc. ✔ [a--b] includes the invalid reversed range a to -
	Invalid Oniguruma syntax		✅	✅	✔ Error
Flags	Supported in top-level flags and flag modifiers
	Ignore case	i	✅	✅	✔ Unicode case folding (same as JS with flag u, v)[8]
	🆚 Dot all	m	✅	✅	✔ Equivalent to JS flag s
	🆕 Extended	x	✅	✅	✔ Unicode whitespace ignored ✔ Line comments with # ✔ Whitespace/comments allowed between a token and its quantifier ✔ Whitespace/comments between a quantifier and the ?/+ that makes it lazy/possessive changes it to a quantifier chain ✔ Whitespace/comments separate tokens (ex: \1 0) ✔ Whitespace and # not ignored in char classes
	Currently supported only in top-level flags
	🆕 Digit is ASCII	D	✅	✅	✔ ASCII \d, \p{Digit}, etc.
	🆕 Space is ASCII	S	✅	✅	✔ ASCII \s, \p{Space}, etc.
	🆕 Word is ASCII[9]	W	✅	✅	✔ ASCII \w, \p{Word}, \b, etc.
	🆕 Text segment mode is grapheme	y{g}	✅	✅	✔ Grapheme based \X, \y
Flag modifiers	Group	(?im-x:…)	✅	✅	✔ Unicode case folding for i ✔ Allows enabling and disabling the same flag (priority: disable) ✔ Allows lone or multiple -
	🆕 Directive	(?im-x)	✅	✅	✔ Continues until end of pattern or group (spanning alternatives)
Compile-time options	ONIG_OPTION_CAPTURE_GROUP		✅	✅	✔ Unnamed captures and numbered calls allowed when using named capture
	ONIG_OPTION_SINGLELINE		✅	✅	✔ ^ → \A ✔ $ → \Z

The table above doesn't include all aspects that Oniguruma-To-ES emulates (including error handling, subpattern details on match results, most aspects that work the same as in JavaScript, and many aspects of non-JavaScript features that work the same in the other regex flavors that support them). Where applicable, Oniguruma-To-ES follows the latest version of Oniguruma (6.9.10).

Footnotes

1. Unicode blocks (which in Oniguruma are specified with an In prefix) are easily emulatable but their character data would significantly increase library weight. They're also rarely used, fundamentally flawed, and arguably unuseful given the availability of Unicode scripts and other properties.
2. With target ES2018, the specific POSIX classes [:graph:] and [:print:] use ASCII-based versions rather than the Unicode versions available for target ES2024 and later, and they result in an error if using strict accuracy.
3. Target ES2018 has limited support for nested, negated character classes.
4. It's not an error for numbered backreferences to come before their referenced group in Oniguruma, but an error is the best path for Oniguruma-To-ES because ① most placements are mistakes and can never match (based on the Oniguruma behavior for backreferences to nonparticipating groups), ② erroring matches the behavior of named backreferences, and ③ the edge cases where they're matchable rely on rules for backreference resetting within quantified groups that are different in JavaScript and aren't emulatable. Note that it's not a backreference in the first place if using \10 or higher and not as many capturing groups are defined to the left (it's an octal or identity escape).
5. Oniguruma's recursion depth limit is 20. Oniguruma-To-ES uses the same limit by default but allows customizing it via the rules.recursionLimit option. Two rare uses of recursion aren't yet supported: overlapping recursions, and use of backreferences when a recursed subpattern contains captures. Patterns that would trigger an infinite recursion error in Oniguruma might find a match in Oniguruma-To-ES (since recursion is bounded), but future versions will detect this and error at transpilation time.
6. Other absence function types aren't yet supported. They start with (?~| and are extremely rare. Note that absence functions behave differently in Oniguruma and Onigmo.
7. Other named callouts aren't yet supported. They use the syntax (*…) and are extremely rare.
8. When using flag i, in extremely rare cases Oniguruma can change the length of matches based on Unicode case conversion rules. The behavior isn't reproduced in this library because ① the rules are applied inconsistently (report) and ② Oniguruma planned to disable all case conversion length changes by default in future versions.
9. Combining flags W and i can result in edge case Oniguruma bugs (report) that aren't reproduced in this library.

❌ Unsupported features

The following throw errors since they aren't yet supported. They're all extremely rare.

• Supportable:
  • Rarely-used character specifiers: Non-A-Za-z with \cx \C-x, meta \M-x \M-\C-x, octal code points \o{…}, and octal encoded bytes ≥ \200.
  • Code point sequences: \x{H H …} \o{O O …}.
  • Flags P (POSIX is ASCII) and y{w} (text segment mode is word), and whole-pattern modifier C (don't capture group).
• Supportable for some uses:
  • Conditionals: (?(…)…), etc.
  • Whole-pattern modifiers I (ignore-case is ASCII) and L (find longest).
  • Named callout (*SKIP).
• Not supportable:
  • Text segment boundaries: \y \Y.
  • Callouts via (?{…}), and most named callouts.

See also the supported features table (above), which describes some additional, rarely-used sub-features that aren't yet supported.

Despite these gaps, ~99.99% of real-world Oniguruma regexes are supported, based on a sample of ~55k regexes used in TextMate grammars. Conditionals were used in three regexes, overlapping recursions in three regexes, and other unsupported features weren't used at all. Some Oniguruma features are so exotic that they aren't used in any public code on GitHub.

🌏 Unicode

Oniguruma-To-ES fully supports mixed case-sensitivity (ex: (?i)a(?-i)a) and handles the Unicode edge cases regardless of JavaScript target.

Oniguruma-To-ES focuses on being lightweight to make it better for use in browsers. This is partly achieved by not including heavyweight Unicode character data, which imposes a few minor/rare restrictions:

• Character class intersection and some uses of nested, negated character classes are unsupported with target ES2018. Use target ES2024 (supported by Node.js 20 and 2023-era browsers) or later if you need support for these features.
• With targets before ES2025, a handful of Unicode properties that target a specific character case (ex: \p{Lower}) can't be used case-insensitively in patterns that contain other characters with a specific case that are used case-sensitively.
  • In other words, almost every usage is fine, including A\p{Lower}, (?i)A\p{Lower}, (?i:A)\p{Lower}, (?i)A(?-i)\p{Lower}, and \w(?i)\p{Lower}, but not A(?i)\p{Lower}.
  • Using these properties case-insensitively is basically never done intentionally, so you're unlikely to encounter this error unless it's catching a mistake.
• Oniguruma-To-ES uses the version of Unicode supported natively by your JavaScript environment. Using Unicode properties via \p{…} that were added in a later version of Unicode than the environment supports results in a runtime error. This is an extreme edge case since modern JavaScript environments support recent versions of Unicode.

🧩 Contributing

Contributions are welcome. See the guide to help you get started.

👀 Similar projects

JsRegex transpiles Ruby regexes to JavaScript. Ruby uses Onigmo, a fork of Oniguruma with similar syntax and behavior. Although JsRegex and this library have important differences, JsRegex might be a better fit for some Ruby projects.

Some high-level JsRegex differences

• It's written in Ruby, so regexes must be pre-transpiled on the server to use them in JavaScript.
• It doesn't always translate edge case behavior differences, include the same level of support for advanced features, or accurately reproduce subpattern results.
• It doesn't include features needed to support TextMate grammars, since that isn't one of its goals.

🏷️ About

Oniguruma-To-ES was created by Steven Levithan and contributors.

If you want to support this project, I'd love your help by contributing improvements (guide), sharing it with others, or sponsoring ongoing development.

MIT License.