Expand description
§pattern-emit
Match a text against a pattern like gr(a|e)y without shipping a regex engine. At compile time this crate turns each constant pattern into a small table of bytes; the program you ship only walks the table.
build reads a pattern as Go’s regexp (RE2) does; build_ecma reads it as ECMA-262 does in Unicode mode with no flags, which is JSON Schema’s pattern. Where the Rust parser, regex-syntax, would read one differently, compile rewrites it when the dialect defines it exactly, and otherwise refuses it and says why.
§Use
Compile side (feature build), once per pattern:
let table = pattern_emit::build(r"gr(a|e)y")?; // Vec<u8>, or a Refusal saying why not
let source = pattern_emit::byte_string(&table); // b"\x00..." to paste into generated codeThe generated code, which needs no feature:
static P0: pattern_emit::Pattern = pattern_emit::Pattern::new(b"...");
let hit = P0.is_match("a grey day")?; // true: matches anywhere, as RE2's MatchStringload checks the table and keeps it, through spin’s Once, so is_match never does; a table nothing loaded gets its check on its first search. The search side is no_std with no allocator. A damaged table, or a search that cannot finish, is an Err(Unusable), never a panic or a trap.
§The guard
The table is a sparse DFA from regex-automata, so both sides must use one regex-automata version: its byte format changes between versions.
Written as RE2 defines them, whatever the Rust parser would do:
\w,\d,\s,\b,\Bare ASCII.\sleaves out\v.- Case folding is Unicode simple folding (
(?i)kmatches the Kelvin sign).
Refused, with the RE2 reading named:
- flags
x,u,R; - class set operations (
&&,--,~~) and classes inside classes, which RE2 reads as literal characters; \<,\>,\b{...},\uand\Uescapes,\p{name=value}and property names RE2 lacks (\p{Emoji});- nested repetition (
a**,a{2}{3}) and counts past 1000.
A test replays Go’s recorded answers: the guard refuses every pattern Go rejects, and every table agrees with Go on every input. tests/fixture/guard.txt lists which patterns Go accepts and the guard declines.
§The ECMA guard
Written as ECMA-262 defines them:
.leaves out\n,\r, U+2028, and U+2029.\sis WhiteSpace and LineTerminator (\v, U+00A0, U+FEFF, the Unicode spaces);\w,\d,\b,\Bare ASCII.^and$anchor the whole text.
Refused, with the ECMA reading named:
- inline flags (
(?i),(?i:…)),\A,\z,\Q…\E,\a,\<,\>,\cX, octal and\0; - POSIX classes (
[[:alpha:]]is literal characters in ECMA), class set operations, and classes inside classes; \x{…}and\Uescapes, escapes of characters that are not syntax, a bare{,}, or], and]first in a class;\pNwithout braces,\p{^…}, and every property but a general category by short name (\p{Lu}): long names, scripts, binary properties like\p{Emoji};(?P<name>…), group names beyond ASCII identifiers, and a name used twice;- lookaround and backreferences, which ECMA has and a table cannot answer;
- a repeated repeat or assertion (
a**,^*).
The guard refuses a surrogate pair written as two \u escapes. Categories follow regex-syntax’s Unicode version, which may differ from a JavaScript engine’s for newly assigned characters.
The same test replays tests/fixture/ecma.json, recorded with Bun (JavaScriptCore) over about 500 patterns and 160 inputs; ecma.txt lists which patterns ECMA accepts and the guard declines. V8 differs on one answer, \B between the halves of a surrogate pair, against the standard.
§Size and cost
A table may take TABLE_CAP bytes: 64 KiB. Ordinary patterns take 1 to 2 KiB; \p{Lu} takes about 30 KiB, .{20} about 17 KiB. What grows a table is Unicode classes (\pL takes about 98 KiB, over the cap), a long .{n}, and counted repetition after an unbounded prefix such as (a|b)*a(a|b){10}. Building stops early on such a blowup instead of running long.
In a WebAssembly module the search costs about 10 KB of code once, against about 67 KB for regex-lite. A search runs 6 to 10 times faster. The check at load runs WebAssembly instructions in proportion to the table: 40 to 340 per byte up to 10 KiB. Tables of 30 KiB and more run up to 805 per byte.
Captures and replacement are not supported; this answers “does it match”.