$re = '/^([^""].+? .+?\.exe)(?!$)/mi';
$str = 'C:\\Program Files (x86)\\InstallShield Installation Information\\{50033799-D5D3-4759-9ABD-589AD04F33C1}\\setup.exe -runfromtemp -l0x0009 -removeonly
C:\\Program Files (x86)\\Some Directory\\Foo.exe
C:\\SomePath\\WithoutSpaces\\Bar.exe -baz
"C:\\Some Path\\With Spaces\\Qux.exe" -quux';
$subst = "\"$1\"";
$result = preg_replace($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