$re = '/^(.*?)((\d[a-z]?\.?)+-.*)$/mi';
$str = 'xorg-fonts-misc-1.0b-1
Xorg-font-bitstream-75dpi-1.0.0-2.i386
Xorg-font-bitstream-100dpi-1.2a-2.arm
Other-Third-Party-1.2.2-1-any';
$subst = "\1 \2";
$result = preg_replace($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