isUnicode property Null safety

Whether this regular expression uses Unicode mode.

In Unicode mode, Dart treats UTF-16 surrogate pairs in the original string as a single code point and will not match each code unit in the pair separately. Otherwise, Dart treats the target string as a sequence of individual code units and does not treat surrogates as special.

In Unicode mode, Dart restricts the syntax of the RegExp pattern, for example disallowing some unescaped uses of restricted regexp characters, and disallowing unnecessary \-escapes ("identity escapes"), which have both historically been allowed in non-Unicode mode. Dart also allows some pattern features, like Unicode property escapes, only in this mode.

var regExp = RegExp(r'^\p{L}$', unicode: true);
print(regExp.hasMatch('a')); // true
print(regExp.hasMatch('b')); // true
print(regExp.hasMatch('?')); // false
print(regExp.hasMatch(r'p{L}')); // false

// U+1F600 (😀), one code point, two code units.
var smiley = '\ud83d\ude00';

regExp = RegExp(r'^.$', unicode: true); // Matches one code point.
print(regExp.hasMatch(smiley)); // true
regExp = RegExp(r'^..$', unicode: true); // Matches two code points.
print(regExp.hasMatch(smiley)); // false

regExp = RegExp(r'^\p{L}$', unicode: false);
print(regExp.hasMatch('a')); // false
print(regExp.hasMatch('b')); // false
print(regExp.hasMatch('?')); // false
print(regExp.hasMatch(r'p{L}')); // true

regExp = RegExp(r'^.$', unicode: false);  // Matches one code unit.
print(regExp.hasMatch(smiley)); // false
regExp = RegExp(r'^..$', unicode: false);  // Matches two code units.
print(regExp.hasMatch(smiley)); // true