$re = '/(Get-Content\s+([\'"]?)C:\\\\bulk\\\\).*?(\.txt\2)/m';
$str = '$line = \'\'
Get-Content C:\\bulk\\XXX.txt |
Select-String -Pattern \'TEMP\' |
ForEach-Object {
#blah blah
}';
$subst = "${1}BBB$3";
$result = preg_replace($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