int indexOf(Pattern pattern, [ int start ])

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.

Source 

int indexOf(Pattern pattern, [int start]);