Question

I have user-submitted text via form that could have multiple blocks of text in pre tags. I need to remove all newlines, without removing the newlines in the PRE tags, and preserve any other user formatting.

Answer 1

You will need to use conditional subpatterns here. Assuming <pre> and </pre> tags are balanced, following code should work for you:

$str = "abc \nfoo\n <pre>123\ndef\nabc\n</pre>qwer\nttt\n bbb";
$p = '~((<pre>)(?(2).*?</pre>)(?:[^\n]*?))*?\n~smi';
$s = preg_replace($p, "$1", $str);
var_dump($s);

OUTPUT

string(42) "abc foo <pre>123
def
abc
</pre>qwerttt bbb"

As you can see \n outside <pre> and </pre> have been removed.

This code searches for 0 or more occurrences of text between <pre> tag in the string and if found then grabs text until </pre> tag. Single iteration of search stops when first \n is found and and then it replaces the matched text with first captured group (i.e. text that came before \n).