fix(r): reject only a real (?< group in exportPattern, not one inside a character class

The linear-regex parser refused any pattern whose text contained "(?<", so
[(?<] and \(?<x were dropped as look-behind. Reject at the group the parser
reaches, which is never inside a class or after a backslash. A real look-behind
or named group is still refused, after any class, with the same linear parse.

gitnexus/src/core/ingestion/languages/r/linear-regex.ts

@@ -325,15 +325,10 @@ const BRACES = /\{(\d+)(?:(,)(\d*))?\}/y;
 class Parser {
   private pos = 0;
   private depth = 0;
-  /** `\k` is an identity escape only when the pattern has no named group. */
-  private readonly hasNamedGroup: boolean;
 
-  constructor(private readonly src: string) {
-    this.hasNamedGroup = src.includes('(?<');
-  }
+  constructor(private readonly src: string) {}
 
   parse(): Node {
-    if (this.hasNamedGroup) reject('named groups and look-behind are unsupported');
     const node = this.parseDisjunction();
     if (this.pos < this.src.length) reject('unmatched )');
     return node;
@@ -414,6 +409,9 @@ class Parser {
     this.pos++; // (
     if (this.peek() === '?') {
       // `(?:` is a plain group; `(?=`, `(?!`, `(?<` and everything else is unsupported or invalid.
+      // Only a `(` the parser reaches here opens a group: one inside a character class
+      // (`[(?<]`) or after a backslash (`\(?<`) is a literal and never gets this far.
+      if (this.peek(1) === '<') reject('named groups and look-behind are unsupported');
       if (this.peek(1) !== ':') reject('look-around and group modifiers are unsupported');
       this.pos += 2;
     }
@@ -543,7 +541,7 @@ class Parser {
         return { code: 0x75 };
       }
       case 'k':
-        // Identity escape (no named groups exist; `parse` already rejected those).
+        // Identity escape (no named groups exist; `parseGroup` rejects those).
         return { code: 0x6b };
       default:
         if (c >= '1' && c <= '9') reject('back-reference or legacy octal escape');

gitnexus/test/unit/r-export-pattern-safety.test.ts

@@ -5,6 +5,7 @@ import { fileURLToPath, pathToFileURL } from 'node:url';
 import { compileRExportPattern } from '../../src/core/ingestion/languages/r/export-pattern.js';
 import {
   compileLinearRegex,
+  compileLinearRegexDetailed,
   MAX_STATES,
 } from '../../src/core/ingestion/languages/r/linear-regex.js';
 
@@ -733,6 +734,48 @@ describe('compileRExportPattern is linear-time on catastrophic patterns', () =>
     }
   });
 
+  describe('"(?<" is only a group opener outside a character class and an escape', () => {
+    it.each([
+      ['[(?<]', ['(', '?', '<'], ['a', '']],
+      ['[^(?<]x', ['ax'], ['(x', '<x']],
+      ['^[(?<]+$', ['(<?', '<'], ['a<', '']],
+      ['[(?<]\\k', ['(k'], ['k']],
+      ['\\(?<x', ['<x', '(<x'], ['x']],
+      ['\\[[(?<]', ['[<', '[('], ['<']],
+      ['[\\]](?:a)', [']a'], ['a']],
+      ['[\\](?<]x', ['(x', ']x', '<x'], ['ax']],
+      ['[[(?<]', ['[', '('], ['a']],
+    ])('compiles %s and matches it as written', (pattern, matching, notMatching) => {
+      const regex = compileLinearRegex(pattern);
+      expect(regex, pattern).not.toBeNull();
+      for (const name of matching) expect(regex?.test(name), `${pattern} ~ ${name}`).toBe(true);
+      for (const name of notMatching) {
+        expect(regex?.test(name), `${pattern} !~ ${name}`).toBe(false);
+      }
+    });
+
+    it.each([
+      ['a real look-behind', '(?<=x)y'],
+      ['a negative look-behind', '(?<!x)y'],
+      ['a named group', '(?<n>a)'],
+      ['after a closed class', '[ab](?<=x)'],
+      ['after a class holding an escaped ]', '[\\]](?<=x)'],
+      ['after an escaped [', '\\[(?<=x)'],
+      ['after a class that holds (?<', '[(?<](?<=x)'],
+      ['inside a group', '(a|(?<=x)b)'],
+    ])('still rejects %s', (_label, pattern) => {
+      expect(compileLinearRegex(pattern), pattern).toBeNull();
+    });
+
+    it('reports a named group or look-behind as such', () => {
+      for (const pattern of ['(?<=x)y', '(?<n>a)']) {
+        const result = compileLinearRegexDetailed(pattern);
+        expect(result.regex).toBeNull();
+        expect(result.reason).toContain('look-behind');
+      }
+    });
+  });
+
   it('bounds the matcher by pattern size, not by the name it is run against', () => {
     // The state count is fixed at compile time and capped, so the work per tested name
     // is at most name length x MAX_STATES whatever the pattern says.