fix(txt): fix chapter detection for bare numbered headings, closes #3570 (#3572)

Three bugs in createChapterRegexps for English txt-to-epub conversion:

1. numberPattern had a capturing group (\d+|...) that leaked single
   digits into split() results instead of chapter titles. Changed to
   non-capturing (?:\d+|...).

2. combinedPattern wrapped the heading in (?:...) so split() consumed
   the title without returning it. Changed to (...) capturing group,
   matching the Chinese regex structure.

3. The prefix (?:^|\n|\s) allowed matching Chapter/Section after any
   whitespace, causing mid-sentence false positives. Changed to
   (?:^|\n) for line-start only.

Added a second-tier regex for bare numbered headings (e.g. "1.1The
Elements of Programming", "1Building Abstractions") commonly found
in academic texts. Dotted numbers allow optional space before title;
single bare digits require immediate title to avoid footnote matches.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/readest-app/src/__tests__/utils/txt-chapter-regex.test.ts

@@ -0,0 +1,117 @@
+import { describe, it, expect } from 'vitest';
+import { TxtToEpubConverter } from '@/utils/txt';
+
+/**
+ * Access private createChapterRegexps via a thin test subclass.
+ */
+class TestableConverter extends TxtToEpubConverter {
+  getChapterRegexps(language: string): RegExp[] {
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    return (this as any).createChapterRegexps(language);
+  }
+}
+
+function splitByRegexps(text: string, regexps: RegExp[]): string[] {
+  for (const regex of regexps) {
+    const parts = text.split(regex);
+    const meaningful = parts.filter((p) => p && p.trim().length > 0);
+    if (meaningful.length > 1) return parts;
+  }
+  return [];
+}
+
+describe('English chapter regexps', () => {
+  const converter = new TestableConverter();
+
+  it('should match "Chapter N" headings', () => {
+    const text =
+      'Intro text\nChapter 1 Getting Started\nContent here\nChapter 2 Next Topic\nMore content';
+    const regexps = converter.getChapterRegexps('en');
+    const parts = splitByRegexps(text, regexps);
+    expect(parts.length).toBeGreaterThan(1);
+  });
+
+  it('should match preface keywords like Foreword and Preface', () => {
+    const text =
+      'Foreword\nSome foreword text\nPreface\nSome preface text\nChapter 1 Start\nContent';
+    const regexps = converter.getChapterRegexps('en');
+    const parts = splitByRegexps(text, regexps);
+    expect(parts.length).toBeGreaterThan(3);
+  });
+
+  it('should match bare dotted-number headings like 1.1Title', () => {
+    const text = [
+      'Preface text here',
+      '1Building Abstractions',
+      'Chapter 1 content here',
+      '1.1The Elements of Programming',
+      'Section content here about elements',
+      '1.1.1Expressions',
+      'Details about expressions',
+      '1.2Procedures and Processes',
+      'More content about procedures',
+    ].join('\n');
+    const regexps = converter.getChapterRegexps('en');
+    const parts = splitByRegexps(text, regexps);
+    expect(parts.length).toBeGreaterThan(3);
+  });
+
+  it('should match dotted-number headings with space before title', () => {
+    const text = [
+      'Intro',
+      '1.1 The Elements of Programming',
+      'Content',
+      '1.2 Procedures and Processes',
+      'More content',
+    ].join('\n');
+    const regexps = converter.getChapterRegexps('en');
+    const parts = splitByRegexps(text, regexps);
+    expect(parts.length).toBeGreaterThan(1);
+  });
+
+  it('should not match bare numbers in code or data lines', () => {
+    // Numbers like "12.7" or "314.159" within content should NOT be treated as headings
+    const text = [
+      '1.1The Elements of Programming',
+      'The value is 12.7 and also 314.159',
+      '62.8318',
+      '1.2Naming and the Environment',
+      'More content',
+    ].join('\n');
+    const regexps = converter.getChapterRegexps('en');
+    const parts = splitByRegexps(text, regexps);
+    // Should split on 1.1... and 1.2... but not on 12.7, 314.159, 62.8318
+    const headings = parts.filter((p) => p && /^\d+\.\d/.test(p.trim()));
+    for (const h of headings) {
+      // Every matched heading should start with a dotted number followed by a letter
+      expect(h.trim()).toMatch(/^\d+(\.\d+)*[A-Z]/);
+    }
+  });
+
+  it('should match single-digit chapter heading like 1Title', () => {
+    const text =
+      '1Building Abstractions with Procedures\nContent\n2Building Abstractions with Data\nMore';
+    const regexps = converter.getChapterRegexps('en');
+    const parts = splitByRegexps(text, regexps);
+    expect(parts.length).toBeGreaterThan(1);
+  });
+
+  it('should not match footnote-like lines with digit-space-lowercase', () => {
+    // Lines like "1 The Lisp..." or "2 The two dialects..." are footnotes, not headings
+    const text = [
+      '1.1The Elements of Programming',
+      'Some content here',
+      '1 The Lisp 1 Programmers Manual appeared in 1960',
+      '2 The two dialects in which most major Lisp programs were written',
+      '3 One such special application was a breakthrough',
+      '1.2Naming and the Environment',
+      'More content',
+    ].join('\n');
+    const regexps = converter.getChapterRegexps('en');
+    const parts = splitByRegexps(text, regexps);
+    // Should split on 1.1... and 1.2... but NOT on "1 The Lisp...", "2 The two...", "3 One such..."
+    const meaningful = parts.filter((p) => p && p.trim().length > 0);
+    // With 2 headings we expect ~5 parts (pre, heading1, content, heading2, content)
+    expect(meaningful.length).toBeLessThanOrEqual(5);
+  });
+});

apps/readest-app/src/__tests__/utils/txt.test.ts

@@ -389,16 +389,16 @@ describe('createChapterRegexps — structure', () => {
     expect(api.createChapterRegexps('zh').length).toBe(2);
   });
 
-  it('should produce one regexp for English', () => {
+  it('should produce two regexps for English', () => {
     const api = getApi();
-    expect(api.createChapterRegexps('en').length).toBe(1);
+    expect(api.createChapterRegexps('en').length).toBe(2);
   });
 
-  it('should produce one regexp for any non-zh language', () => {
+  it('should produce two regexps for any non-zh language', () => {
     const api = getApi();
-    expect(api.createChapterRegexps('fr').length).toBe(1);
-    expect(api.createChapterRegexps('ja').length).toBe(1);
-    expect(api.createChapterRegexps('de').length).toBe(1);
+    expect(api.createChapterRegexps('fr').length).toBe(2);
+    expect(api.createChapterRegexps('ja').length).toBe(2);
+    expect(api.createChapterRegexps('de').length).toBe(2);
   });
 
   it('Chinese first regex should be case-insensitive with unicode flag', () => {

apps/readest-app/src/utils/txt.ts

@@ -655,7 +655,7 @@ export class TxtToEpubConverter {
       'Afterword',
     ];
 
-    const numberPattern = String.raw`(\d+|(?:[IVXLCDM]{2,}|V|X|L|C|D|M)\b)`;
+    const numberPattern = String.raw`(?:\d+|(?:[IVXLCDM]{2,}|V|X|L|C|D|M)\b)`;
     const dotNumberPattern = String.raw`\.\d{1,4}`;
     const titlePattern = String.raw`[^\n]{0,50}`;
 
@@ -670,9 +670,16 @@ export class TxtToEpubConverter {
       .map((k) => String.raw`${k}(?:[:.\-–—]?\s*${titlePattern})?`)
       .join('|');
 
-    const combinedPattern = String.raw`(?:^|\n|\s)(?:${normalChapterPattern}|${prefacePattern})(?=\s|$)`;
+    const combinedPattern = String.raw`(?:^|\n)(${normalChapterPattern}|${prefacePattern})(?=\s|$)`;
     chapterRegexps.push(new RegExp(combinedPattern, 'gi'));
 
+    // Second-tier: bare numbered headings like "1.1The Elements" or "1Building Data"
+    // Dotted numbers (1.1, 1.2.3) allow an optional space before the title.
+    // Single bare digits (1, 2) require the title to start immediately (no space)
+    // to avoid matching footnotes like "1 The Lisp...".
+    const numberedHeadingPattern = String.raw`(?:^|\n)(\d+\.\d+(?:\.\d+)* ?[A-Z][^\n]{0,80}|\d+[A-Z][^\n]{0,80})`;
+    chapterRegexps.push(new RegExp(numberedHeadingPattern, 'g'));
+
     return chapterRegexps;
   }