$re = '/^.*:.*\K:.*$/m';
$str = '52152525121512@gmail.com:161361361:1251136136
532353472626326@gmail.com:2361251251225:-
126125252155@gmail.com:215125151251252:135263623,13631535315
';
$subst = "";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php