$re = '/(?!^)(?!$)/mi';
$str = ' E9:F18 M17 M17 X1:H8
F105 S34 X1:H8
F31:X1 Q1:X1 W3 ';
$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