Question

I have the following string:

<?foo?> <?bar?> <?baz?> hello world <?/?> <?/?> <?/?>

I need a regular expression to convert it into

<?foo?> <?bar?> <?baz?> hello world <?/baz?> <?/bar?> <?/foo?>

The following code works for non-recursive tags:

$x=preg_replace_callback('/.*?<\?\/\?>/',function($x){
    return preg_replace('/(.*<\?([^\/][\w]+)\?>)(.*?)(<\?\/?\?>)/s',
          '\1\3<?/\2?>',$x[0]);
},$str);

Answer 1

You can’t do this with regular expressions. You need to write a parser!

So create a stack (an array where you add and remove items from the end. use array_push() array_pop() ).

Iterate through the tags, pushing known opening tags on the stack.

When you come to a closing tag, pop the stack and that will tell you the tag you need to close.