Strings

Replace First, All, and Mapped

Dart's String is immutable, so every replace method returns a new string instead of mutating the receiver. replaceFirst(pat, sub) swaps only the first occurrence of a pattern, replaceAll(pat, sub) swaps every occurrence, and replaceAllMapped(regex, fn) runs a callback on each RegExp match and joins the callback's return values. The original text is unchanged across all three calls.

Program

Play the program to swap the first red, swap every red, then wrap each word in stars with a regex callback.

string_replace.dart
Replay: real traced execution (multi-file project)
void main() {
  var text = 'red red blue';
  var oneSwap = text.replaceFirst('red', 'RED');
  var allSwap = text.replaceAll('red', 'RED');
  var stars = text.replaceAllMapped(RegExp(r'\w+'), (m) => '*${m.group(0)}*');
  print('$oneSwap | $allSwap | $stars');
}

  1. text ← red red blue

    1void main() {2  var text = 'red red blue';3  var oneSwap = text.replaceFirst('red', 'RED');
    values this stepred red bluetext

  2. oneSwap ← RED red blue

    2var text = 'red red blue';3var oneSwap = text.replaceFirst('red', 'RED');4var allSwap = text.replaceAll('red', 'RED');
    values this stepRED red blueoneSwapred red bluetext

  3. allSwap ← RED RED blue

    3var oneSwap = text.replaceFirst('red', 'RED');4var allSwap = text.replaceAll('red', 'RED');5var stars = text.replaceAllMapped(RegExp(r'\w+'), (m) => '*${m.group(0)}*');
    values this stepRED RED blueallSwapred red bluetext

  4. stars ← *red* *red* *blue*

    4var allSwap = text.replaceAll('red', 'RED');5var stars = text.replaceAllMapped(RegExp(r'\w+'), (m) => '*${m.group(0)}*');6print('$oneSwap | $allSwap | $stars');
    values this step*red* *red* *blue*starsred red bluetext

  5. print('$oneSwap | $allSwap | $stars');

    5  var stars = text.replaceAllMapped(RegExp(r'\w+'), (m) => '*${m.group(0)}*');6  print('$oneSwap | $allSwap | $stars');7}
    outputRED red blue | RED RED blue | *red* *red* *blue*
    values this stepRED red blueoneSwapRED RED blueallSwap*red* *red* *blue*stars

Follow the Replacements

  1. text starts as red red blue.
  2. replaceFirst changes only the first red to RED.
  3. replaceAll changes both red words to RED.
  4. replaceAllMapped wraps every word in stars.
  5. The program prints RED red blue | RED RED blue | *red* *red* *blue*. | expression | result | | --- | --- | | replaceFirst | RED red blue | | replaceAll | RED RED blue | | replaceAllMapped | red red blue |
replaceFirst `text.replaceFirst(pat, sub)` returns a new `String` with only the first occurrence of `pat` replaced by `sub`; later matches are kept.
replaceAll `text.replaceAll(pat, sub)` returns a new `String` with every occurrence replaced. The original `text` is unchanged because `String` is immutable.
replaceAllMapped `text.replaceAllMapped(regex, fn)` runs `fn` on each `RegExp` match and joins the returned strings. `fn` receives a `Match`; `m.group(0)` is the whole match.

Exercise: string_replace.dart

Reproduce RED red blue | RED RED blue | *red* *red* *blue*, then trace which replacement changes one red, both reds, and every word.