[Flutter] Use dynamic (variable) string as regex pattern in dart

i am trying to pass variable in regex language DART

`betweenLenth(val, field, [min = 4, max = 20]) {
final RegExp nameExp = new RegExp(r'^\w{" + min + "," + max + "}$');
if (!nameExp.hasMatch(val))
return field + " must be between $min - $max characters ";
}`


thanks

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