Replace text in ASS subtitles ignoring formatting

ass-text-replacer.js

function replaceAssText (source, replacerFn) {
  // Detect all non-dialogue tokens and mark those with null characters
  const lines = source.replace(/\0/g, '').split('\n')
  const nonReplaceableTokens = []
  const replaceableTokens = []
  let nonTextAccumulator = ''

  for (let line of lines) {
    if (!line.startsWith('Dialogue: ')) {
      nonTextAccumulator += line + '\n'
      continue
    }

    const lineParts = line.split(',')
    const nonTextLine = lineParts.slice(0, 9).join(',')
    nonTextAccumulator += nonTextLine + ','
    nonReplaceableTokens.push(nonTextAccumulator)
    nonTextAccumulator = ''

    const dialogueText = lineParts.slice(9).join(',')
    const nonTextTokens = dialogueText.match(/\{.*?\}|\\N/g)
    const textTokens = dialogueText.split(/\{.*?\}|\\N/g)
    if (nonTextTokens) {
      for (let token of textTokens) replaceableTokens.push(token)
      for (let token of nonTextTokens) nonReplaceableTokens.push(token)
    } else {
      replaceableTokens.push(dialogueText)
    }
    replaceableTokens[replaceableTokens.length - 1] += '\n'
  }

  nonReplaceableTokens.push(nonTextAccumulator)
  replaceableText = '\0' + replaceableTokens.join('\0') + '\0'
  
  // Replace text with null characters using the replacer function
  replaceableText = replacerFn(replaceableText)

  // Place back the non-dialogue tokens
  let replaceIndex = 0
  return replaceableText.replace(/\0/g, () => nonReplaceableTokens[replaceIndex++])
}

// Function usage
const result = replaceAssText(assText, (text) => text.replace(/"([\s\S]*?)"/g, '“$1”'))
console.log(result)

Important: the replacer function must not add or remove null characters.