Regular expression to check if two first words are same

Question

For example:

$s1 = \"Test Test the rest of string\"
$s2 = \"Test the rest of string\"

I would like to match positively $s1 but

Answer 1

Not working everywhere, see the comments...

^([^\b]+)\b\1\b
^(\B+)\b\1\b

Gets the first word, and matches if the same word is repeated again after a word boundary.