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 not $s2, because first word in $s1 is the same as second. Word 'Test' is example, regular expression should work on any words.

Answer 1

if(preg_match('/^(\w+)\s+\1\b/',$input)) {
  // $input has same first two words.
}

Explanation:

^    : Start anchor
(    : Start of capturing group
 \w+ : A word
)    : End of capturing group
\s+  : One or more whitespace
\1   : Back reference to the first word
\b   : Word boundary

Answer 2

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

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