$re = '/(?<=(["\']\b))(?:(?=(\\\\?))\2.)*?(?=\1)/';
$str = 'Hello Colm "This is a test"
"This is also an NL test"
\'And this has an escaped quote don\\\'t in it \' Blue Boy
"This has a single quote \' but doesn\\\' end the quote as it started with double quotes"
"Foo Bar" "Another Value" something else';
$subst = "Subbed";
$result = preg_replace($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