#engine/regexp

Odin port of QuickJS-ng's libregexp: a full ECMAScript regular-expression parser and bytecode compiler, plus a bounded backtracking interpreter. It replaces the C engine and leans on the language — tagged opcodes, interval character sets, caller-supplied allocators, an explicit (non-recursive) backtrack stack — so it builds for windows_amd64, linux_amd64 and freestanding_wasm32 with no libc. The browser uses it for URLPattern (src/url/pattern_match.odin: a component's regular expression, with the v flag).

import rx "wb:engine/regexp"

#API

re, err := rx.compile(pattern, flags, allocator)   // flags: rx.Flags bit_set
defer rx.destroy(&re)
if err.message != "" { /* compile error at err.pos */ }

caps: rx.Captures
st := rx.exec_search(&re, rx.input_utf16(units), start, &caps, limits, allocator)
// st is .Match / .No_Match / .Out_Of_Steps / .Stack_Overflow
if st == .Match {
    whole := caps.groups[0]        // {start, end} in code-unit offsets, or {-1,-1} if unset
    idx, ok := rx.named_group_index(&re, "year")
}

• Flags d g i m s u v y — rx.parse_flags("gims") → rx.Flags; rx.flags_to_string.
• Input is a slice of code units, either UTF-16 (rx.input_utf16, JS semantics, offsets are UTF-16 indices) or UTF-8 (rx.input_utf8 / rx.input_utf8_string, offsets are byte indices). Surrogate pairs combine only in unicode mode.
• exec tries exactly one start offset; exec_search scans forward from start (and respects the sticky y flag, which forbids advancing).
• Captures are reported in code-unit offsets; group 0 is the whole match. Allocate on an arena and free the whole match at once.

#Limits (configurable)

The interpreter is a flat dispatch loop with an explicit backtrack stack, so a pathological pattern cannot overflow the native stack. Two budgets bound it, exposed as preferences (regexp.exec.max_steps, regexp.exec.max_stack) and overridable per call via Exec_Limits:

• max_steps — opcode dispatches before the match fails with .Out_Of_Steps (default 10,000,000).
• max_stack — saved choice points before .Stack_Overflow (default 1,000,000).

tools builds read the defaults; a host reads them from the registry with rx.limits_from_prefs.

#Grammar covered

Alternation; capturing, non-capturing and named groups (?<name>…); the assertions ^ $ \b \B; positive and negative lookahead and lookbehind; back references, numbered \1 and named \k<name>; character classes with ranges, the class escapes \d \D \w \W \s \S, and \p{…} / \P{…} (general category, binary property, Script, Script_Extensions); quantifiers * + ? {n} {n,} {n,m} with an optional lazy ?; the escape forms \f \n \r \t \v \0 \cX \xHH \uHHHH \u{…}; the v flag's nested classes, escaped reserved punctuators (\:, \&…) and chains of set operations A && B && C, A -- B -- C; and the Annex B web-reality leniencies when neither u nor v is set (lone {, ], } as literals; invalid back references as octal/identity escapes).

#How it works

• parser.odin / parse_*.odin build an AST (two-pass: a prescan counts groups and collects names so forward references resolve).
• compile*.odin lower the AST to bytecode (the AST lives in an arena of the compilation's own): quantifiers become split/goto/loop forms with an empty-match guard (Push_Char_Pos / Check_Advance) on every iteration past the minimum, so (a*)* cannot loop forever and (.*)? that matched nothing leaves its group undefined; classes resolve to ucd.Char_Range interval sets, case-folded once at compile time when i is set; a lookbehind body is compiled in reverse so a dedicated backward interpreter walks it.
• vm.odin / vm_behind.odin / vm_read.odin are the interpreter: forward and reverse dispatch, UTF-8/UTF-16 reading in both directions, word-boundary and back-reference matching, and the lookaround sub-runner (captures from a positive lookaround persist; a failed or negative one leaves none).

#Judged against the C reference

wrentool engine rx-diff builds a native library from QuickJS-ng 0.17's libregexp.c + libunicode.c and runs this engine and the C side over a literal corpus, tens of thousands of random fuzz patterns, and a speed benchmark, reporting every divergence.

Latest run (Unicode 16.0.0, seed 12345, 30,000 fuzz patterns):

• ~2,016,000 checks. Zero divergences on regexp compile+exec, normalization, and property / category / script ranges (boundary-exact). The only mismatches (~3,700) are scalar Unicode predicates (is_id_start/is_id_continue/is_cased/is_case_ignorable) on a handful of code points where QuickJS 0.17's bundled libunicode-table.h differs from UCD 16.0.0 — a data-version skew, not an engine difference. Re-pin the generator to QuickJS's exact UCD revision to close it.
• Speed: 2.1×–4.6× slower than the C reference on five typical patterns (~0.2–3 µs/exec). The gap is the capture snapshot taken at each choice point; a copy-on-write capture scheme would narrow it.
• Reference bug found: QuickJS-ng 0.17's lre_exec corrupts the heap when a group combined with +/* (e.g. (a|b)+) is executed many times in one process. Reproduced in pure C, independent of this engine; the harness guards the C calls and the fuzz generator avoids the trigger for the differential pass while still fuzzing the Odin engine over the unrestricted grammar in -NoC mode (where it is robust).

Run it:

build\wrentool engine rx-diff -Seed 12345 -Fuzz 30000      # differential + speed
build\wrentool engine rx-diff -NoC -Fuzz 200000            # Odin-only soundness fuzz

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