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regex-partial-match

A zero-dependency regular expression transform for partial matching, enabling validation of incomplete input strings against regex patterns.

🧩 Problem statement

Unlike C/C++ (via PCRE/PCRE2, RE2, Boost.Regex), Python (via third party regex module) or Java (via hitEnd), Javascript has no canonical / innate partial-matching for regular expressions.

📖 Overview

This library transforms regular expressions to best-effort support partial matching, allowing you to test if an incomplete string could potentially match the full pattern. This is particularly useful for real-time input validation, autocomplete systems, progressive form validation, stream chunk matching, etc.

As a side effect of the parse this requires, each PartialMatchRegExp also exposes a features set naming the syntactic constructs its pattern uses — useful for consumers that need to reason about a pattern without writing their own regex parser. For many features, a simple search in the source would be insufficient.

Based on an algorithm created by Lucas Trzesniewski, re-created for NPM via ISC license, with permission.

📦 Installation

npm install regex-partial-match

🚀 Usage

Basic Usage

import PartialMatchRegExp from "regex-partial-match";

const pattern = /^hello world/;
const partial = new PartialMatchRegExp(pattern);

partial.test("h"); // true - could match
partial.test("hello"); // true - could match
partial.test("hello world"); // true - full match
partial.test("goodbye"); // false - cannot match

Extending RegExp.prototype

import "regex-partial-match/extend";

const partial = /^hello world/.toPartialMatchRegex();

partial.test("hel"); // true

⚙️ How It Works

The library transforms a regular expression by wrapping each atomic element in a non-capturing group with a disjunction to a true-end-of-input sentinel ($(?![\s\S])1):

/abc/ → /(?:a|$(?![\s\S]))(?:b|$(?![\s\S]))(?:c|$(?![\s\S]))/

This allows the pattern to match prefixes of the original pattern, enabling validation of incomplete input.

Since the library accepts only valid regular expressions 2, this enables the algorithm to make lots of unguarded assumptions about the source of the expression.

The library has been stress-tested with various regular expression features in isolation, and some in likely combination, but obviously it's an unbounded test space, and syntactically valid regular expressions nevertheless support contradictory patterns e.g.

  • /\b\B/ - impossible to match both a word boundary and a non-word boundary
  • /$^/ - end cannot come before start
  • x{2}? - lazy quantifiers are mutually exclusive to fixed-length assertions

Such combinations have not been tested.

Note

See Partial Match Parity for full details on how the library compares to reference implementations

Patterns with backreferences

Backreferences cannot be handled by the |$(?![\s\S]) transform alone because they are atomic — \1 must match the entire captured string or fail, and its length is only known at runtime. PartialMatchRegExp first tries a full match natively, but that native result only wins outright if nothing earlier in the input could still be a viable partial — a cheap bound check settles that without needing to resolve the backreference's actual value, so the common case (no earlier partial exists) stays fast. Otherwise it runs a "capture scan": a variant of the pattern with each backreference swapped for a lazy (?:[\s\S]*?) wildcard, so the group it depends on can still capture against a partial input — matching anything, or nothing at all, without needing to already know the backreference's value.

Whatever that scan captures (or leaves undefined, if the group hasn't been reached yet) is then used to build a fresh partial-matching regex for this specific input, expanding the backreference character-by-character from the captured value with the same per-atom transform as the rest of the pattern. See docs/backreferences.md for the full algorithm, including the prefer-longer post-processing that preserves correct captures for groups inside quantifiers.

✅ Supported Features

🚫 Unsupported Features

The following regex features are not currently supported:

🌐 Browser Compatibility

The library is compiled to ES2015 (ECMAScript 6). Certain regular expression features naturally require newer environments:

⚠️ Caveats

.test() behaviour and non-matching results from .exec() and .match()

For unanchored patterns (no ^ and not using the y flag), the library produces an expression that always matches an empty string at the true end of the input — see How It Works. Feasibly, this is the start of a new partial match.

Hence:

/x/.test("a") === false; /* untransformed regex */
/(?:x|$(?![\s\S]))/.test("a") === true; /* new PartialMatchRegExp(/x/), internally */

To mitigate, a start anchor (^) can prevent the engine from scanning forward to match the empty-string fallback at the end of the input:

/* new PartialMatchRegExp(/^x/) matches as if it were /^(?:x|$(?![\s\S]))/ */
/^(?:x|$(?![\s\S]))/.test("") === true;
/^(?:x|$(?![\s\S]))/.test("x") === true;
/^(?:x|$(?![\s\S]))/.test("a") === false;

Caution

In multiline mode, ^ still matches at the start of the string and immediately after each \n, so the transformed regex can attempt the empty-string fallback at the start of any line — but, since the fallback requires strict end-of-input, it only succeeds if that line start is also genuinely where the input ends:

/^(?:x|$(?![\s\S]))/m.test("x") === true;
/^(?:x|$(?![\s\S]))/m.test("a\n") === true;  /* '^' matches after '\n', and input truly ends there */
/^(?:x|$(?![\s\S]))/m.test("a\nb") === false; /* '^' matches after '\n', but "b" remains — not genuine end-of-input */

The y flag prevents matching ahead from the lastIndex (defaulting to 0 for a new RegExp):

/(?:x|$(?![\s\S]))/y.test("x") === true;
/(?:x|$(?![\s\S]))/y.test("a") === false;

Caution

See caveats re: resetting lastIndex when incrementally matching

On this basis, .test() should be used with caution, and a match of an empty string at the true end of the input should instead be considered "no match", if validating that which came before.

e.g.

/(?:x|$(?![\s\S]))/.exec("a"); // ['', index: 1, input: "a", groups: undefined];
"a".match(/(?:x|$(?![\s\S]))/); // ['', index: 1, input: "a", groups: undefined];

Note

A more ergonomic test() / exec() output was explored, but proved a complex problem space.

Backreferences

PartialMatchRegExp supports partial matching of backreferences (\1, \k<name>) — see Patterns with backreferences above and docs/backreferences.md for the algorithm. A backreference is inherently atomic — \1 must match the complete captured text or fail — but the library resolves what each group captured from a partial input and expands the backreference into per-character partial form so matching can still proceed character-by-character in the common case.

The following cases remain atomic (full native value or exactly at true end of input, no mid-value partial matching):

  • Backreferences inside lookbehinds and negative lookarounds. These are verbatim contexts — the value a lookbehind or negative lookahead requires must be fully present or fully absent, so there's no partial-prefix position to expand into.
  • \k<name> with no named capturing groups in the pattern. Annex B tolerates this as the literal characters k<a>, but it's still treated as if it were a named backreference and matched atomically — "k" and "k<" will not partially match. A \k not immediately followed by a well-formed <name> reference is treated as the literal k and partially matches as usual.
  • A backreference whose captured value can't be determined from a partial input. This only affects the backreference site itself; it's strictly better than rejecting the input outright, and never accepts anything unsound.

Prefix-ambiguous top-level alternation

When a pattern uses top-level alternation where one branch is a strict prefix of another (e.g. ^(ab)\1|^(abc)\2), the internal capture scan may select the shorter branch — because it uses (?:[\s\S]*?) which accepts zero characters — causing the final partial regex to fail for inputs that are valid prefixes of the longer branch. In such cases exec returns null even though the input is a valid partial match:

const partial = new PartialMatchRegExp(/^(ab)\1|^(abc)\2/);

partial.test("abca"); // false — but "abca" is a valid prefix of "abcabc" via the second branch

Tip

If alternate branches share a prefix, list the longer one first. The capture scan tries branches in order and stops at the first that accepts the partial input, so putting the longer branch first ensures it's the one selected:

const partial = new PartialMatchRegExp(/^(abc)\2|^(ab)\1/);

partial.test("abca"); // true

See docs/backreferences.md for why this happens (the internal capture scan resolving the wrong alternative first).

Positive Lookbehinds

Whilst forming a match, a positive lookbehind must match in entirety, for the pattern to match. This is inherent in the concept of non-matching groups, since they are not match-worthy themselves, but just qualify matching atoms.

e.g.

/(?<=foo)bar/;

"f" through "foo" is not a match, but "foob" is.

Surrogate Pair Matching

In unicode-aware mode (u flag), only whole astral characters are supported. Partial matching of individual surrogate pairs is not supported. For example, /😀/u will match the complete emoji character, but not the first surrogate pair in isolation. Hence, if partially matching a byte stream, be sure to pipe via a TextDecoder first.

Sticky Flag (y)

The sticky flag is fully supported for its intended use case: scanning within a single fixed string. Partial matches are found only at lastIndex; the engine does not scan forward, and lastIndex advances on success or resets to 0 on failure — exactly as native sticky regexes behave.

import PartialMatchRegExp from "regex-partial-match";

const partial = new PartialMatchRegExp(/hello/y);

partial.lastIndex = 2;
partial.test("xyhello"); // true  — partial match at position 2
partial.test("xyworld"); // false — no match at position 2, no forward scan
partial.lastIndex = 2;
partial.test("xyhel"); // true  — partial prefix "hel" at position 2

Limitation — progressive input validation: Because a successful match advances lastIndex, testing a sequence of growing strings against the same instance does not work as expected:

const partial = new PartialMatchRegExp(/hello/y);

partial.test("h");   // true,  lastIndex → 1
partial.test("he");  // false — sticky requires a match at position 1 of "he",
                     //         but "e" is not a valid start of the pattern
partial.test("hel"); // true (lastIndex was reset to 0 by the previous failure)

There is no way to distinguish "scanning forward in the same string" from "testing a new, longer string", so this cannot be fixed in code. For progressive input validation, use a regex without the y flag and always test against the full input so far.

The gy flag combination is also fully supported: exec()/test() behave as sticky, while match(), matchAll(), replace(), and replaceAll() iterate via exec() as global — matching the language specification.

"String properties"

As with surrogate pair matching, grapheme clusters / string properties can only match atomically.

Hence, [\p{RGI_Emoji_Flag_Sequence}] will match 🇺🇳 as a whole, but not as the individual code points of which it's comprised.

In v mode expressions, where [\q{abc}] syntax is used in isolation (rather than its canonical use-case as a subtraction/intersection of another character class), this will also only match entirely or not at all. i.e. abc can match, but not partially.

💡 Examples

Form Validation

import PartialMatchRegExp from "regex-partial-match";

const emailPattern = /^[a-z0-9._%+-]+@[a-z0-9.-]+\.[a-z]{2,}$/i;
const partial = new PartialMatchRegExp(emailPattern);

function validateEmail(input) {
  return partial.test(input) ? "valid" : "invalid";
}

validateEmail("user"); // 'valid' - could become valid
validateEmail("user@"); // 'valid' - could become valid
validateEmail("user@example"); // 'valid' - could become valid
validateEmail("user@example.com"); // 'valid' - complete match
validateEmail("@@invalid"); // 'invalid' - cannot match

Autocomplete

import PartialMatchRegExp from "regex-partial-match";

const commandPattern = /^(help|quit|save|load)/;
const partial = new PartialMatchRegExp(commandPattern);

function getSuggestions(input) {
  return partial.test(input) ? "valid prefix" : "no suggestions";
}

getSuggestions("h"); // 'valid prefix'
getSuggestions("hel"); // 'valid prefix'
getSuggestions("help"); // 'valid prefix'
getSuggestions("xyz"); // 'no suggestions'

Stream Processing

import PartialMatchRegExp from "regex-partial-match";

// Process streaming data with pattern matching at chunk boundaries
const pattern = /\{"[^"]+":"[^"]+"\}/; // Match JSON objects
const partial = new PartialMatchRegExp(pattern);
let buffer = "";

function processChunk(chunk) {
  buffer += chunk;
  const matches = [];

  // Extract complete matches
  let match;
  while ((match = pattern.exec(buffer))) {
    matches.push(match[0]);
    buffer = buffer.slice(match.index + match[0].length);
  }

  // Discard buffer if it cannot possibly complete
  if (buffer && !partial.test(buffer)) {
    buffer = "";
  }

  return matches;
}

processChunk('{"na'); // [] - partial, buffer: '{"na'
processChunk('me":"Jo'); // [] - partial, buffer: '{"name":"Jo'
processChunk('hn"}{"age":'); // ['{"name":"John"}'] - buffer: '{"age":'
processChunk("25}"); // ['{"age":25}'] - buffer: ''
processChunk("invalid{"); // [] - discarded, buffer: ''

Useful for parsing log files, network streams, or any chunked data where records may be split across boundaries.

🔌 API

new PartialMatchRegExp(pattern: RegExp | string, flags?: string)

Extends RegExp. An instance behaves like a normal RegExp — instanceof RegExp is true, and .test(), .exec(), .match(), .matchAll(), .replace(), etc. all work as expected — but also matches any input string that is a valid prefix of the original pattern, in addition to full matches.

Available via the default entry point of the package.

Parameters:

  • pattern - A RegExp instance, or a pattern source string (as accepted by the RegExp constructor)
  • flags - A flags string, used only when pattern is a string (as accepted by the RegExp constructor)

Returns:

  • A PartialMatchRegExp instance that matches partial strings of the original pattern

RegExp.prototype.toPartialMatchRegex(): PartialMatchRegExp

When using import 'regex-partial-match/extend', this method is added to RegExp.prototype.

Returns:

  • A new PartialMatchRegExp that matches partial strings, created from the RegExp instance the method was called on.

PartialMatchRegExp.prototype.features: ReadonlySet<RegexFeature>

Building the partial-match regex requires walking the entire source pattern once. As a side effect of that same walk, each instance records which syntactic constructs its pattern actually uses, exposed as a features set — no separate scan of the source is performed to produce it.

This is useful for consumers building on top of PartialMatchRegExp who need to reason about which constructs a specific pattern uses, without writing their own regex parser to find out. Two concrete cases:

  • Flagging patterns likely to hit one of the caveats documented above. For example, a pattern combining backreference with lookbehind, negativeLookahead, or negativeLookbehind is a candidate for the atomic-backreference caveat; one combining backreference with disjunction is a candidate for the prefix-ambiguous top-level alternation caveat. A consumer accepting user-supplied patterns can surface a warning instead of letting the edge case surprise someone later.
  • Restricting which constructs a product surface allows. e.g. a system that only wants to accept "simple" patterns (no lookaround, no backreferences) from untrusted input can check features against an allow-list and reject the rest, without needing to hand-roll that check against the raw pattern source.
import PartialMatchRegExp from "regex-partial-match";

const partial = new PartialMatchRegExp(/^[a-z]+(?<domain>\.[a-z]+)\1/);

partial.features; // Set { "startAnchor", "characterClass", "quantifier", "namedGroup", "capturingGroup", "backreference" }
partial.features.has("backreference"); // true

RegexFeature is a string union, exported alongside PartialMatchRegExp:

Feature Matches Notes
patternCharacter An ordinary literal character
startAnchor Top-level ^
endAnchor Top-level $
wordBoundary Top-level \b
nonWordBoundary Top-level \B
lookahead (?=...)
negativeLookahead (?!...)
lookbehind (?<=...)
negativeLookbehind (?<!...)
backreference \1
namedBackreference \k<name>
namedGroup (?<name>...) Always accompanied by capturingGroup — see below
capturingGroup (...), including named groups
nonCapturingGroup (?:...)
modifierGroup (?ims:...)
modifierGroupWithRemoval (?ims-ims:...) Mutually exclusive with modifierGroup
characterClass [...]
nestedCharacterClass [...[...]...] v flag only
classIntersection && inside a character class v flag only
classSubtraction -- inside a character class v flag only
disjunction |
quantifier *, +, ?, {n}, {n,}, {n,m}
unicodePropertyEscape \p{...}, \P{...} u/v flag only — otherwise tagged otherEscape
characterClassEscape \d, \D, \w, \W, \s, \S
controlEscape \f, \n, \r, \t, \v
controlLetterEscape \cX
hexEscapeSequence \xXX
unicodeEscapeSequence \uXXXX, \u{...}
otherEscape Any other \X, e.g. \., \0

Two things worth knowing about how these tags line up with the grammar:

  • One ECMA-262 production can map to several tags. Assertion alone covers ^, $, \b, \B, and all four lookarounds — features splits it by whichever discriminant is easiest to read off during the walk (^ vs $, = vs ! after (?<, etc.), since that information is free at the point each construct is recognised.
  • A named capturing group always carries both namedGroup and capturingGroup. The grammar treats a capturing group with a name and one without as the same production (( GroupSpecifier? Disjunction )), not two, so both tags are added together.

📜 License

ISC License - see LICENSE file for details.

🙌 Credits

Algorithm created by Lucas Trzesniewski.

🤝 Contributing

Contributions are welcome! Please open an issue or pull request on GitHub.

🔗 Related projects

Project Description
incr-regex-package Incremental regex matcher
dfa Compiles a regular expression like syntax to fast deterministic finite automata, which could be used to partial match?
refa Can convert regular expressions to an Abstract Syntax Tree, which might afford partial-match capability?
@eslint-community/regexpp A regular expression parser for ECMAScript with AST generation and visitor implementation
Regex+ template literal, transforming native regular expressions
Awesome Regex Curated list of tools, tutorials, libraries, and other resources, covering all major regex flavours
replace-content-transformer A toolkit for stream content replacement, underpinned by regex-partial-match

Footnotes

  1. A bare $ alone isn't sufficient here: under the m (multiline) flag — including one turned on locally via a (?m:...) modifier — $ also matches immediately before any line terminator, not just the true end of input. That would let a "\n" the source pattern never allowed for be silently accepted as if the input had simply run out, e.g. new PartialMatchRegExp(/^foobar/m) would wrongly accept "foo\nbaz". Appending (?![\s\S]) narrows the disjunction down to strict end-of-input, regardless of multiline state.

    See chromium issue 536420076 for the underlying V8 bug that requires $ to precede (?![\s\S]) rather than using the lookahead alone.

    A shorter option, (?-m:$) — disabling multiline locally so $ means strict end-of-input on its own — also sidesteps the bug and saves a few bytes per atom. However, modifier groups are new enough that support isn't universal, and feature-detecting them would add a fallback branch the test suite can't exercise honestly, since every engine that can realistically be tested against already supports them. ↩

  2. To remain lightweight, no runtime type validation is applied, so non-TypeScript consumers will be reliant on underlying errors thrown if used incorrectly. ↩

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A regular expression transform for partial matching

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