$re = '/\b(\d+)\s*:\s*(\d+)\s*([ap])\.\s*m\./m';
$str = '2:0 a. m.
2:0 p. m.
3:0 p. m.
3:0p.m.
2:0 am';
$subst = "$1:$2 $3m";
$result = preg_replace($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