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

~^(\w+)\s+\1(?:\W|$)~
~^(\pL+)\s+\1(?:\PL|$)~u // unicode variant

\1 is a back reference to the first capturing group.