Dart regular expressions

last modified January 28, 2024

In this article we show how to work with regular expressions in Dart.

Regular expressions are used for text searching and more advanced text manipulation. We find them tools such as grep and sed, text editors such as vi and Emacs, and programming languages.

RegExp is used to define a regular expression. Dart regular expressions have the same syntax and semantics as JavaScript regular expressions.

Dart regex hasMatch

The hasMatch function checks whether the regular expression has a match in the specified string.

main.dart

void main() { var words = <String>[“Seven”, “even”, “Maven”, “Amen”, “eleven”];

var rx = RegExp(r".even");

for (var word in words) { if (rx.hasMatch(word)) { print("${word} does match"); } else { print("${word} does not match"); } } }

In the example, we have five words in a list. We check which words match the .even regular expression.

var rx = RegExp(r".even");

We define a regular expression with RegExp. The dot (.) metacharacter stands for any single character in the text.

for (var word in words) { if (rx.hasMatch(word)) { print("${word} does match"); } else { print("${word} does not match"); } }

We go over the elements of the list and check if the elements match the regular expression with hasMatch.

$ dart main.dart Seven does match even does not match Maven does not match Amen does not match eleven does match

Dart regex alternation

The alternation operator | creates a regular expression with several choices.

main.dart

void main() { var words = [ “Jane”, “Thomas”, “Robert”, “Lucy”, “Beky”, “John”, “Peter”, “Andy” ];

var rx = RegExp(“Jane|Beky|Robert”);

words.forEach((word) { if (rx.hasMatch(word)) { print(“the ${word} matches”); } }); }

We have eight names in the list.

var rx = RegExp(“Jane|Beky|Robert”);

This regular expression looks for “Jane”, “Beky”, or “Robert” strings.

$ dart main.dart the Jane matches the Robert matches the Beky matches

Dart regex subpatterns

Subpatterns are patterns within patterns. Subpatterns are created with () characters.

main.dart

void main() { var words = [ “book”, “bookshelf”, “bookworm”, “bookcase”, “bookish”, “bookkeeper”, “booklet”, “bookmark” ];

var rx = RegExp(r"^book(worm|mark|keeper)?$");

for (var word in words) { if (rx.hasMatch(word)) { print("${word} does match"); } else { print("${word} does not match"); } } }

We find all words that matches the subpatterns.

var rx = RegExp(r"^book(worm|mark|keeper)?$");

The regular expression uses a subpattern. It matches bookworm, bookmark, bookkeeper, and book words.

$ dart main.dart book does match bookshelf does not match bookworm does match bookcase does not match bookish does not match bookkeeper does match booklet does not match bookmark does match

Dart RegExp allMatches

The allMatches function finds all matches of the given regular exressions.

main.dart

void main() { var text = ‘The Sun was shining; I went for a walk.’;

var rx = RegExp(r"\w+");

var found = rx.allMatches(text); print(“There are ${found.length} words”);

found.forEach((match) { print("${match.group(0)} ${match.start}:${match.end}"); }); }

In the example, we find all words in the text.

var rx = RegExp(r"\w+");

The regular expression matches all words; where a word is defined as a one or more characters.

print(“There are ${found.length} words”);

We print the number of words.

found.forEach((match) { print("${match.group(0)} ${match.start}:${match.end}"); });

We iterate over all found matches and print them and their indexes.

$ dart mains.dart There are 9 words The 0:3 Sun 4:7 was 8:11 shining 12:19 I 21:22 went 23:27 for 28:31 a 32:33 walk 34:38

Dart regex capturing groups

Round brackets are used to create capturing groups. This allows us to apply a quantifier to the entire group or to restrict alternation to a part of the regular expression.

main.dart

void main() { var sites = [“webcode.me”, “zetcode.com”, “freebsd.org”, “netbsd.org”];

var rx = RegExp(r"(\w+).(\w+)");

for (var site in sites) { var matches = rx.allMatches(site); matches.forEach((match) { print(match.group(0)); print(match.group(1)); print(match.group(2)); }); print(’———————-’); } }

In the example, we divide the domain names into two parts by using groups.

var rx = RegExp(r"(\w+).(\w+)");

We define two groups with parentheses. The dot character is escaped because it is used in the literal meaning.

var matches = rx.allMatches(site);

We find all matches with allMatches.

print(match.group(0)); print(match.group(1)); print(match.group(2));

The groups are accessed via the group function. The default whole match is available with match.group(0).

$ dart main.dart webcode.me webcode me

zetcode.com zetcode com

freebsd.org freebsd org

netbsd.org netbsd org

Source

Dart Regex - language reference

In this article we covered Dart regular expressions.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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