Returns the position of the last match pattern in this string, searching backward starting at start, inclusive:
var string = 'Dartisans';
string.lastIndexOf('a'); // 6
string.lastIndexOf(new RegExp(r'a(r|n)')); // 6 Returns -1 if pattern could not be found in this string.
string.lastIndexOf(new RegExp(r'DART')); // -1
The start must be non-negative and not greater than length.
int lastIndexOf(Pattern pattern, [int start]);
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https://api.dart.dev/stable/2.5.0/dart-core/String/lastIndexOf.html