How to use RegEx in Dart? - regex

Question

In a Flutter application, I need to check if a string matches a specific RegEx. However, the RegEx I copied from the JavaScript version of the app always returns false in the Flutter app. I verified on regexr that the RegEx is valid, and this very RegEx is already being used in the JavaScript application, so it should be correct.
Any help is appreciated!
RegEx : /^WS{1,2}:\/\/\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}:56789/i
Test Code :
RegExp regExp = new RegExp(
r"/^WS{1,2}:\/\/\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}:56789/i",
caseSensitive: false,
multiLine: false,
);
print("allMatches : "+regExp.allMatches("WS://127.0.0.1:56789").toString());
print("firstMatch : "+regExp.firstMatch("WS://127.0.0.1:56789").toString());
print("hasMatch : "+regExp.hasMatch("WS://127.0.0.1:56789").toString());
print("stringMatch : "+regExp.stringMatch("WS://127.0.0.1:56789").toString());
Output :
allMatches : ()
firstMatch : null
hasMatch : false
stringMatch : null

Answer 1

This is a more general answer for future viewers.
Regex in Dart works much like other languages. You use the RegExp class to define a matching pattern. Then use hasMatch() to test the pattern on a string.
Examples
Alphanumeric
final alphanumeric = RegExp(r'^[a-zA-Z0-9]+$');
alphanumeric.hasMatch('abc123'); // true
alphanumeric.hasMatch('abc123%'); // false
Hex colors
RegExp hexColor = RegExp(r'^#?([0-9a-fA-F]{3}|[0-9a-fA-F]{6})$');
hexColor.hasMatch('#3b5'); // true
hexColor.hasMatch('#FF7723'); // true
hexColor.hasMatch('#000000z'); // false
Extracting text
final myString = '25F8..25FF ; Common # Sm [8] UPPER LEFT TRIANGLE';
// find a variable length hex value at the beginning of the line
final regexp = RegExp(r'^[0-9a-fA-F]+');
// find the first match though you could also do `allMatches`
final match = regexp.firstMatch(myString);
// group(0) is the full matched text
// if your regex had groups (using parentheses) then you could get the
// text from them by using group(1), group(2), etc.
final matchedText = match?.group(0); // 25F8
There are some more examples here.
See also:
Extracting text from a string with regex groups in Dart

Answer 2

I think you tried to include options in the raw expression string while you already have it as parameters to RegEx ( /i for case insensitivity is declared as caseSensitive: false).
// Removed /i at the end
// Removed / in front - Thanks to Günter for warning
RegExp regExp = new RegExp(
r"^WS{1,2}:\/\/\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}:56789",
caseSensitive: false,
multiLine: false,
);
print("allMatches : "+regExp.allMatches("WS://127.0.0.1:56789").toString());
print("firstMatch : "+regExp.firstMatch("WS://127.0.0.1:56789").toString());
print("hasMatch : "+regExp.hasMatch("WS://127.0.0.1:56789").toString());
print("stringMatch : "+regExp.stringMatch("WS://127.0.0.1:56789").toString());
Gives:
allMatches : (Instance of '_MatchImplementation')
firstMatch : Instance of '_MatchImplementation'
hasMatch : true
stringMatch : WS://127.0.0.1:56789