regex

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

Supported: flags d g i m s u v y; named groups (incl. 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; every early error. Backtracking uses an explicit choice-point stack and undo trail, so pathological patterns never recurse on the Nim stack.

API

Convenience layer:

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.

Low-level layer (what a host engine uses, zero-copy):

rxCompile(units: seq[int], flags, err): int	compile; program index or -1
rxBindWide(ptr UncheckedArray[uint16]) / rxBindNarrow(ptr UncheckedArray[char])	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); reported index or -1
rxMem[0 ..< 2*ncaps]	captures after a successful run
rxProgs[pi]	ncaps, names (per group, UTF-16 units), 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.

Test

nimony c -p:src -p:../unicode/src tests/test_regex.nim   # then run the binary