Strings

RegExp Basics

RegExp(pattern) compiles a regular expression from a string. Three common entry points cover most learner cases: hasMatch(text) returns a bool that answers whether the pattern hits anywhere, firstMatch(text) returns the first Match (or null) and m.group(0) reads the whole match, and allMatches(text) returns every match as an Iterable<Match> that can be transformed with the usual map/join.

Program

Play the program to compile r'b\w+', check that it matches cat bat bird, read the first hit, and join every hit with a comma.

regex_basics.dart
Replay: real traced execution (multi-file project)
void main() {
  var text = 'cat bat bird';
  var pattern = r'b\w+';
  var re = RegExp(pattern);
  var ok = re.hasMatch(text);
  var first = re.firstMatch(text)!.group(0);
  var all = re.allMatches(text).map((m) => m.group(0)).join(',');
  print('$ok $first $all');
}

  1. text ← cat bat bird

    1void main() {2  var text = 'cat bat bird';3  var pattern = r'b\w+';
    values this stepcat bat birdtext

  2. pattern ← b\w+

    2var text = 'cat bat bird';3var pattern = r'b\w+';4var re = RegExp(pattern);
    values this stepb\w+pattern

  3. re ← RegExp(b\w+)

    3var pattern = r'b\w+';4var re = RegExp(pattern);5var ok = re.hasMatch(text);
    values this stepRegExp(b\w+)reb\w+pattern

  4. ok ← true

    4var re = RegExp(pattern);5var ok = re.hasMatch(text);6var first = re.firstMatch(text)!.group(0);
    values this steptrueokRegExp(b\w+)recat bat birdtext

  5. first ← bat

    5var ok = re.hasMatch(text);6var first = re.firstMatch(text)!.group(0);7var all = re.allMatches(text).map((m) => m.group(0)).join(',');
    values this stepbatfirstRegExp(b\w+)recat bat birdtext

  6. all ← bat,bird

    6var first = re.firstMatch(text)!.group(0);7var all = re.allMatches(text).map((m) => m.group(0)).join(',');8print('$ok $first $all');
    values this stepbat,birdallRegExp(b\w+)recat bat birdtext

  7. print('$ok $first $all');

    7  var all = re.allMatches(text).map((m) => m.group(0)).join(',');8  print('$ok $first $all');9}
    outputtrue bat bat,bird
    values this steptrueokbatfirstbat,birdall

Follow the Matches

  1. text starts as cat bat bird.
  2. The pattern b\w+ looks for words that start with b.
  3. hasMatch is true because matches exist.
  4. The first match is bat.
  5. All matches join as bat,bird, so the output is true bat bat,bird. | check | result | | --- | --- | | any match? | true | | first match | bat | | all matches | bat,bird |
RegExp `RegExp(pattern)` compiles a regular expression from a string. A raw string `r'...'` keeps backslashes literal, so `b\w+` reads cleanly.
hasMatch and firstMatch `re.hasMatch(text)` returns `bool`. `re.firstMatch(text)` returns the first `Match` (or `null`); `m.group(0)` is the whole match text.
allMatches `re.allMatches(text)` returns every match as an `Iterable<Match>`, so `.map((m) => m.group(0)).join(',')` collects each hit into a comma-separated string.

Exercise: regex_basics.dart

Reproduce true bat bat,bird, then trace why b\\w+ first finds bat and then finds bat,bird as all matches.