Lua's string.gsub in TypeScript

How to use

Install via yarn add locutus and import: import { gsub } from 'locutus/lua/string/gsub'.

Or with CommonJS: const { gsub } = require('locutus/lua/string/gsub')

Use a bundler that supports tree-shaking so you only ship the functions you actually use. Vite, webpack, Rollup, and Parcel all handle this. For server-side use this is less of a concern.

Examples

These examples are extracted from test cases that automatically verify our functions against their native counterparts.

#	code	expected result
1	gsub('hello world', 'l', 'L')	['heLLo worLd', 3]
2	gsub('123-456', '(%d+)', '[%1]', 1)	['[123]-456', 1]
3	gsub('abc', '%a', 'x')	['xxx', 3]

Notes

• Supports common Lua pattern classes and capture substitutions like %1 in replacement strings.

Here's what our current TypeScript equivalent to Lua's string.gsub looks like.

TSTypeScript JSJavaScript TSStandalone TS JSStandalone JS

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const LUA_PATTERN_ESCAPES: Record<string, string> = {
  a: '[A-Za-z]',
  A: '[^A-Za-z]',
  c: '[\\x00-\\x1F\\x7F]',
  C: '[^\\x00-\\x1F\\x7F]',
  d: '[0-9]',
  D: '[^0-9]',
  l: '[a-z]',
  L: '[^a-z]',
  p: '[!-/:-@[-`{-~]',
  P: '[^!-/:-@[-`{-~]',
  s: '\\s',
  S: '\\S',
  u: '[A-Z]',
  U: '[^A-Z]',
  w: '[A-Za-z0-9]',
  W: '[^A-Za-z0-9]',
  x: '[A-Fa-f0-9]',
  X: '[^A-Fa-f0-9]',
  z: '\\x00',
  Z: '[^\\x00]',
}

const LUA_CHAR_CLASS_ATOMS: Record<string, string> = {
  a: '[A-Za-z]',
  A: '[^A-Za-z]',
  c: '[\\x00-\\x1F\\x7F]',
  C: '[^\\x00-\\x1F\\x7F]',
  d: '[0-9]',
  D: '[^0-9]',
  l: '[a-z]',
  L: '[^a-z]',
  p: '[!-/:-@\\[-`\\{-~]',
  P: '[^!-/:-@\\[-`\\{-~]',
  s: '\\s',
  S: '\\S',
  u: '[A-Z]',
  U: '[^A-Z]',
  w: '[A-Za-z0-9]',
  W: '[^A-Za-z0-9]',
  x: '[A-Fa-f0-9]',
  X: '[^A-Fa-f0-9]',
  z: '\\x00',
  Z: '[^\\x00]',
}

function escapeRegexLiteral(char: string): string {
  return /[\\^$.*+?()[\]{}|/]/.test(char) ? `\\${char}` : char
}

function escapeRegexClassLiteral(char: string): string {
  return char === '\\' || char === ']' || char === '^' || char === '-' ? `\\${char}` : char
}

function translateLuaCharClass(chars: string): string {
  let negate = false
  let source = chars
  if (source.startsWith('^')) {
    negate = true
    source = source.slice(1)
  }

  const atoms: string[] = []

  for (let index = 0; index < source.length; ) {
    const current = parseLuaClassToken(source, index)
    const hyphen = source[index + current.length] ?? ''
    const nextIndex = index + current.length + 1
    const canRange =
      hyphen === '-' &&
      nextIndex < source.length &&
      current.literal !== null &&
      current.rangeable &&
      source[nextIndex] !== ']'

    if (canRange) {
      const right = parseLuaClassToken(source, nextIndex)
      if (current.literal !== null && right.literal !== null && right.rangeable) {
        const leftLiteral = current.literal
        const rightLiteral = right.literal
        atoms.push(`[${escapeRegexClassLiteral(leftLiteral)}-${escapeRegexClassLiteral(rightLiteral)}]`)
        index = nextIndex + right.length
        continue
      }
    }

    atoms.push(current.atom)
    index += current.length
  }

  const union = atoms.length === 1 ? (atoms[0] ?? '') : `(?:${atoms.join('|')})`
  return negate ? `(?:(?!${union})[\\s\\S])` : union
}

function parseLuaClassToken(
  source: string,
  index: number,
): { atom: string; length: number; literal: string | null; rangeable: boolean } {
  const char = source[index] ?? ''
  if (char === '%') {
    const next = source[index + 1] ?? ''
    const mapped = LUA_CHAR_CLASS_ATOMS[next]
    if (mapped) {
      return { atom: mapped, length: 2, literal: null, rangeable: false }
    }
    return { atom: escapeRegexLiteral(next), length: 2, literal: next, rangeable: false }
  }

  return {
    atom: escapeRegexLiteral(char),
    length: 1,
    literal: char,
    rangeable: char !== '\\',
  }
}

function canApplyLuaQuantifier(parts: string[]): boolean {
  if (parts.length === 0) {
    return false
  }

  const last = parts[parts.length - 1] ?? ''
  return last !== '(' && last !== '^' && last !== '$'
}

function translateLuaPattern(pattern: string): RegExp {
  const parts: string[] = []

  for (let i = 0; i < pattern.length; i++) {
    const char = pattern[i] ?? ''

    if (char === '%') {
      const next = pattern[i + 1] ?? ''
      if (next === '') {
        parts.push('%')
        continue
      }

      if (next === 'b' || next === 'f') {
        throw new Error('gsub(): unsupported Lua pattern feature')
      }

      i++
      parts.push(LUA_PATTERN_ESCAPES[next] ?? escapeRegexLiteral(next))
      continue
    }

    if (char === '[') {
      let classBody = ''
      let closed = false
      i++
      if (i < pattern.length && pattern[i] === '^') {
        classBody += '^'
        i++
      }
      if (i < pattern.length && pattern[i] === ']') {
        classBody += ']'
        i++
      }
      while (i < pattern.length && pattern[i] !== ']') {
        classBody += pattern[i] ?? ''
        i++
      }
      if (i < pattern.length && pattern[i] === ']') {
        closed = true
      }
      if (!closed) {
        throw new Error('gsub(): malformed Lua pattern')
      }
      parts.push(translateLuaCharClass(classBody))
      continue
    }

    if (char === '.') {
      parts.push('[\\s\\S]')
      continue
    }

    if (char === '-' || char === '*' || char === '+' || char === '?') {
      if (canApplyLuaQuantifier(parts)) {
        parts[parts.length - 1] += char === '-' ? '*?' : char
      } else {
        parts.push(escapeRegexLiteral(char))
      }
      continue
    }

    if (char === '^') {
      parts.push(i === 0 ? '^' : '\\^')
      continue
    }

    if (char === '$') {
      parts.push(i === pattern.length - 1 ? '$' : '\\$')
      continue
    }

    if ('()'.includes(char)) {
      parts.push(char)
      continue
    }

    parts.push(escapeRegexLiteral(char))
  }

  return new RegExp(parts.join(''), 'g')
}

function applyLuaReplacement(template: string, match: string, captures: string[]): string {
  let out = ''

  for (let i = 0; i < template.length; i++) {
    const char = template[i] ?? ''
    if (char !== '%') {
      out += char
      continue
    }

    const next = template[i + 1] ?? ''
    if (next === '%') {
      out += '%'
      i++
      continue
    }

    if (/\d/.test(next)) {
      i++
      if (next === '0') {
        out += match
      } else {
        const capture = captures[Number(next) - 1]
        if (capture === undefined) {
          throw new Error(`invalid capture index %${next}`)
        }
        out += capture
      }
      continue
    }

    out += next
    i++
  }

  return out
}

export function gsub(source: string, pattern: string, replacement: string, limit?: number): [string, number] {
  //      discuss at: https://locutus.io/lua/gsub/
  // parity verified: Lua 5.4
  //     original by: Kevin van Zonneveld (https://kvz.io)
  //          note 1: Supports common Lua pattern classes and capture substitutions like %1 in replacement strings.
  //       example 1: gsub('hello world', 'l', 'L')
  //       returns 1: ['heLLo worLd', 3]
  //       example 2: gsub('123-456', '(%d+)', '[%1]', 1)
  //       returns 2: ['[123]-456', 1]
  //       example 3: gsub('abc', '%a', 'x')
  //       returns 3: ['xxx', 3]

  const value = String(source)
  const regex = translateLuaPattern(String(pattern))
  const maxReplacements = limit === undefined ? Number.POSITIVE_INFINITY : Math.max(0, Math.trunc(Number(limit)))
  let count = 0

  if (maxReplacements === 0) {
    return [value, 0]
  }

  const out = value.replace(regex, (...args) => {
    const match = String(args[0] ?? '')
    if (count >= maxReplacements) {
      return match
    }

    count++
    const captures = args.slice(1, -2).map((item) => String(item ?? ''))
    return applyLuaReplacement(String(replacement), match, captures)
  })

  return [out, count]
}

const LUA_PATTERN_ESCAPES = {
  a: '[A-Za-z]',
  A: '[^A-Za-z]',
  c: '[\\x00-\\x1F\\x7F]',
  C: '[^\\x00-\\x1F\\x7F]',
  d: '[0-9]',
  D: '[^0-9]',
  l: '[a-z]',
  L: '[^a-z]',
  p: '[!-/:-@[-`{-~]',
  P: '[^!-/:-@[-`{-~]',
  s: '\\s',
  S: '\\S',
  u: '[A-Z]',
  U: '[^A-Z]',
  w: '[A-Za-z0-9]',
  W: '[^A-Za-z0-9]',
  x: '[A-Fa-f0-9]',
  X: '[^A-Fa-f0-9]',
  z: '\\x00',
  Z: '[^\\x00]',
}

const LUA_CHAR_CLASS_ATOMS = {
  a: '[A-Za-z]',
  A: '[^A-Za-z]',
  c: '[\\x00-\\x1F\\x7F]',
  C: '[^\\x00-\\x1F\\x7F]',
  d: '[0-9]',
  D: '[^0-9]',
  l: '[a-z]',
  L: '[^a-z]',
  p: '[!-/:-@\\[-`\\{-~]',
  P: '[^!-/:-@\\[-`\\{-~]',
  s: '\\s',
  S: '\\S',
  u: '[A-Z]',
  U: '[^A-Z]',
  w: '[A-Za-z0-9]',
  W: '[^A-Za-z0-9]',
  x: '[A-Fa-f0-9]',
  X: '[^A-Fa-f0-9]',
  z: '\\x00',
  Z: '[^\\x00]',
}

function escapeRegexLiteral(char) {
  return /[\\^$.*+?()[\]{}|/]/.test(char) ? `\\${char}` : char
}

function escapeRegexClassLiteral(char) {
  return char === '\\' || char === ']' || char === '^' || char === '-' ? `\\${char}` : char
}

function translateLuaCharClass(chars) {
  let negate = false
  let source = chars
  if (source.startsWith('^')) {
    negate = true
    source = source.slice(1)
  }

  const atoms = []

  for (let index = 0; index < source.length; ) {
    const current = parseLuaClassToken(source, index)
    const hyphen = source[index + current.length] ?? ''
    const nextIndex = index + current.length + 1
    const canRange =
      hyphen === '-' &&
      nextIndex < source.length &&
      current.literal !== null &&
      current.rangeable &&
      source[nextIndex] !== ']'

    if (canRange) {
      const right = parseLuaClassToken(source, nextIndex)
      if (current.literal !== null && right.literal !== null && right.rangeable) {
        const leftLiteral = current.literal
        const rightLiteral = right.literal
        atoms.push(`[${escapeRegexClassLiteral(leftLiteral)}-${escapeRegexClassLiteral(rightLiteral)}]`)
        index = nextIndex + right.length
        continue
      }
    }

    atoms.push(current.atom)
    index += current.length
  }

  const union = atoms.length === 1 ? (atoms[0] ?? '') : `(?:${atoms.join('|')})`
  return negate ? `(?:(?!${union})[\\s\\S])` : union
}

function parseLuaClassToken(source, index) {
  const char = source[index] ?? ''
  if (char === '%') {
    const next = source[index + 1] ?? ''
    const mapped = LUA_CHAR_CLASS_ATOMS[next]
    if (mapped) {
      return { atom: mapped, length: 2, literal: null, rangeable: false }
    }
    return { atom: escapeRegexLiteral(next), length: 2, literal: next, rangeable: false }
  }

  return {
    atom: escapeRegexLiteral(char),
    length: 1,
    literal: char,
    rangeable: char !== '\\',
  }
}

function canApplyLuaQuantifier(parts) {
  if (parts.length === 0) {
    return false
  }

  const last = parts[parts.length - 1] ?? ''
  return last !== '(' && last !== '^' && last !== '$'
}

function translateLuaPattern(pattern) {
  const parts = []

  for (let i = 0; i < pattern.length; i++) {
    const char = pattern[i] ?? ''

    if (char === '%') {
      const next = pattern[i + 1] ?? ''
      if (next === '') {
        parts.push('%')
        continue
      }

      if (next === 'b' || next === 'f') {
        throw new Error('gsub(): unsupported Lua pattern feature')
      }

      i++
      parts.push(LUA_PATTERN_ESCAPES[next] ?? escapeRegexLiteral(next))
      continue
    }

    if (char === '[') {
      let classBody = ''
      let closed = false
      i++
      if (i < pattern.length && pattern[i] === '^') {
        classBody += '^'
        i++
      }
      if (i < pattern.length && pattern[i] === ']') {
        classBody += ']'
        i++
      }
      while (i < pattern.length && pattern[i] !== ']') {
        classBody += pattern[i] ?? ''
        i++
      }
      if (i < pattern.length && pattern[i] === ']') {
        closed = true
      }
      if (!closed) {
        throw new Error('gsub(): malformed Lua pattern')
      }
      parts.push(translateLuaCharClass(classBody))
      continue
    }

    if (char === '.') {
      parts.push('[\\s\\S]')
      continue
    }

    if (char === '-' || char === '*' || char === '+' || char === '?') {
      if (canApplyLuaQuantifier(parts)) {
        parts[parts.length - 1] += char === '-' ? '*?' : char
      } else {
        parts.push(escapeRegexLiteral(char))
      }
      continue
    }

    if (char === '^') {
      parts.push(i === 0 ? '^' : '\\^')
      continue
    }

    if (char === '$') {
      parts.push(i === pattern.length - 1 ? '$' : '\\$')
      continue
    }

    if ('()'.includes(char)) {
      parts.push(char)
      continue
    }

    parts.push(escapeRegexLiteral(char))
  }

  return new RegExp(parts.join(''), 'g')
}

function applyLuaReplacement(template, match, captures) {
  let out = ''

  for (let i = 0; i < template.length; i++) {
    const char = template[i] ?? ''
    if (char !== '%') {
      out += char
      continue
    }

    const next = template[i + 1] ?? ''
    if (next === '%') {
      out += '%'
      i++
      continue
    }

    if (/\d/.test(next)) {
      i++
      if (next === '0') {
        out += match
      } else {
        const capture = captures[Number(next) - 1]
        if (capture === undefined) {
          throw new Error(`invalid capture index %${next}`)
        }
        out += capture
      }
      continue
    }

    out += next
    i++
  }

  return out
}

export function gsub(source, pattern, replacement, limit) {
  //      discuss at: https://locutus.io/lua/gsub/
  // parity verified: Lua 5.4
  //     original by: Kevin van Zonneveld (https://kvz.io)
  //          note 1: Supports common Lua pattern classes and capture substitutions like %1 in replacement strings.
  //       example 1: gsub('hello world', 'l', 'L')
  //       returns 1: ['heLLo worLd', 3]
  //       example 2: gsub('123-456', '(%d+)', '[%1]', 1)
  //       returns 2: ['[123]-456', 1]
  //       example 3: gsub('abc', '%a', 'x')
  //       returns 3: ['xxx', 3]

  const value = String(source)
  const regex = translateLuaPattern(String(pattern))
  const maxReplacements = limit === undefined ? Number.POSITIVE_INFINITY : Math.max(0, Math.trunc(Number(limit)))
  let count = 0

  if (maxReplacements === 0) {
    return [value, 0]
  }

  const out = value.replace(regex, (...args) => {
    const match = String(args[0] ?? '')
    if (count >= maxReplacements) {
      return match
    }

    count++
    const captures = args.slice(1, -2).map((item) => String(item ?? ''))
    return applyLuaReplacement(String(replacement), match, captures)
  })

  return [out, count]
}

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