Returns the position of the first match of pattern in this string, starting at start, inclusive:
var string = 'Dartisans';
string.indexOf('art'); // 1
string.indexOf(new RegExp(r'[A-Z][a-z]')); // 0 Returns -1 if no match is found:
string.indexOf(new RegExp(r'dart')); // -1
start must be non-negative and not greater than length.
int indexOf(Pattern pattern, [int start]);
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https://api.dart.dev/stable/2.5.0/dart-core/String/indexOf.html