regex — an ECMAScript regex engine for Nimony

▶️ Try aoughwl/regex live in the Playground — clones the repo into the in-browser IDE, no install.

regex is an ECMAScript (ES2025) regular-expression engine for Nimony, operating on UTF-16 code units: the parser, compiler and backtracking matcher of the aowljs-engine JavaScript engine, with no dependency on any JS value model. It depends on unicode for property tables and case folding.

• What it supports
• Use it
• API
• Build / test
• Used by

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What it supports

• flags d g i m s u v y
• named groups, including duplicate names across alternatives
• lookahead and lookbehind; numbered and named backreferences
• modifiers (?i:...)
• \p{...} General_Category / Script / Script_Extensions / binary properties
• /v class set expressions (&&, --, nested classes, \q{...}, properties of strings such as RGI_Emoji)
• case-insensitive matching per ECMA-262 Canonicalize
• Annex B legacy syntax outside u/v mode, and every early error

Backtracking uses an explicit choice-point stack and undo trail, so pathological patterns never recurse on the Nim stack.

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Use it

import regex
var err = ""
let re = compileRegex(r"(?<y>\d{4})-(?<m>\d\d)", "u", err)   # -1 and err on SyntaxError
let caps = execAt(re, "on 2024-05!", 0)   # @[3, 10, 3, 7, 8, 10]; @[] = no match
echo groupIndex(re, "m")                 # 2

execAt takes a seq[uint16] (or a UTF-8 string, converted with toUnits) and returns 2 * (groupCount + 1) UTF-16 offsets, -1 for a group that did not participate. With flag y it matches only at start, otherwise it searches forward.

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API

The low-level layer is what a host engine uses, zero-copy:

rxCompile(units: seq[int], flags, err): int	compile; program index or -1
rxBindWide / rxBindNarrow	bind the subject (UTF-16, or Latin-1 bytes)
rxRunN(pi, n, start): bool	match at exactly start
rxScanN(pi, n, from, fullUnicode): int	search forward (with first-unit / first-matcher skip); index or -1
rxMem[0 ..< 2*ncaps]	captures after a successful run
rxProgs[pi]	ncaps, names, u, fbits (1 g, 2 y, 4 d, 8 u/v), ...
rxMatch1, rxReadF, rxCanon, rxIsSyntaxChar, rsHas	single-character helpers

The matcher keeps global state: it is neither reentrant nor thread-safe.

---

Build / test

git clone https://github.com/aoughwl/regex
git clone https://github.com/aoughwl/unicode
cd regex
nimony c -p:src -p:../unicode/src tests/test_regex.nim   # then run the binary

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Used by

• aowljs-engine — its RegExp is this library.