From regex

Status: built (2026-11-03). fromRegex() translates a regex where it can prove the result is equivalent, and otherwise refuses it, with the reason. Over tjs-lang's own source, 67% of regexes translate, and every translation agrees with its regex on generated inputs; the largest refusal is not backtracking but looking behind (\b, multiline ^, lookbehind: 21%). The numbers are in reports/from-regex-2026-11-03.md.

import { fromRegex } from 'tosijs-pattern'
import { show } from 'tosijs-pattern/demo'

const version = fromRegex(/^v(?<major>\d+)\.(?<minor>\d+)$/)
let refusal
try {
  fromRegex(/\d+5/)
} catch (e) {
  refusal = `${e.code}: ${e.message}`
}
show(preview, version.describe(), version.parse('v2.13').captures, refusal)

A translation has the regex's SEARCH semantics (find, findAll, replaceAll, …). Unnamed groups capture as $1, $2; named groups by name. A refusal is a RegexRefusal with a stable code, the offset at in the source, and a suggestion. Pattern reads code points, so a regex WITHOUT the u flag (which reads UTF-16 units) can differ on characters beyond the BMP, and nowhere else.

People, and language models especially, write regex fluently. So the plan is to keep accepting regex as INPUT while only ever running Pattern. A host (the tjs-lang AsyncJS VM is the first) takes a regex literal, translates it at compile time, and runs the result. A regex that cannot be translated faithfully never runs; the author gets a message that names the problem and the Pattern piece to use instead.

Why translation is not just syntax

Regex backtracks; Pattern does not. Syntax maps one-to-one, but the meaning only matches when the backtracking could never have changed the outcome:

• \d+5 on "1235": the regex gives back the 5 and matches; sequence(oneOrMore(digit), '5') does not.
• a|ab followed by c, on "abc": the regex tries ab after a fails later; oneOf('a', 'ab') does not.

So the translator proves, per site, that possessive and ordered semantics give the same answer, and refuses when it cannot. The proof is a local check:

1. A repetition is safe when nothing that can follow it can start with a character it could have consumed (the "first set" of what follows does not overlap the repeated item's). \d+- is safe (a digit is never a -); \d+5 is not.
2. An alternation is safe when no option can match where an earlier one already matched a prefix of it (in practice, when their first characters differ, or the longer comes first, which the translator can reorder only when that is provably equivalent).
3. Where the check fails, the translator looks for the STRUCTURE the backtracking was standing in for, and names it:
   • .*?X and .*X before a literal become until(X).
   • "(?:[^"\\]|\\.)*" becomes quoted('"', { escape: '\\' }).
   • [^,]*(?:,[^,]*)* becomes separatedBy(…, ',').
4. Anything else is refused: "\d+5: the repetition can consume what follows it, so this regex only matches by retrying. Say where the digits stop: …".

The rest of the mapping

What it found

"Looks behind" is \b, ^ with the m flag, and lookbehind: each asks about the character BEFORE the position, which Pattern cannot yet. A precededBy piece would recover most of it; that is the next decision.

How much will translate? The plan, before measuring

A census of the 7,926 distinct regex literals in a large node_modules (2026-11-02; mostly developer tooling, so lexer-heavy) found: quantifiers in 76%, character classes 60%, alternation 45%, capture groups 40%, lookahead 15%, lazy quantifiers 7%, backreferences 3%, lookbehind 0.5%. Only about 13% were simple enough for plain string methods.

That population is not the one that matters most. The first job of the translator is to report, over a corpus, what share translates and why the rest is refused, run over:

1. that census (the script is scripts/regex-census.ts once it is moved here);
2. regexes language models write for typical agent tasks (extract fields from model output, validate a date or an email, split records, clean whitespace), collected with tjs-lang's grokkability harness;
3. the regexes in tjs-lang's own compiler (about 350 sites), which are also the first migration target.

Every translated regex is checked by a DIFFERENTIAL test: the regex and its translation run on generated inputs, and must agree on every one. That test is the only place the host RegExp is allowed.

Pattern as a way to understand regexes

The translator gives something useful even where no one runs the result: any regex, read aloud. Paste ^(?:[a-z0-9._]+)@[a-z0-9-]+(?:\.[a-zA-Z]+)+$ and get "the start of the input, then one or more characters in .0-9_a-z, then "@", …", or, where it is refused, the precise reason (where it would backtrack, and what it was standing in for). That is an explainer for the regexes people inherit, review, and copy from answers they do not understand: the same audience the "Regexes are Hard" survey found (70% of developers say regexes are less readable than other code). It wants a page on the doc site where you paste a regex and see the pattern, its description, and its railroad diagram.