I think your Regular Expression is unnecessarily complex.

Indeed it is, but it does give the specified output in the root node.

Besides it chokes on a string like this: "sp Hello there! spelling spoiler spooky asp wizard! sp \n Hey hey sp How are you? sp".

Unfortunately so which is why I recommended it to be made more generic (i.e not rely on space being around 'sp').

A simpler and better solution would be

That would be nice but unfortunately it gives this incorrect output

$VAR1 = [
          '',
          ' Hello there ',
          '
 Hey hey ',
          ' How are you? '
        ];
[download]

As outlined below it's splitting the string on 'sp' as opposed to grabbing the text between it (as though the first 'sp' were a <sp> and the second a </sp> and so on)

01              2             3
sp Hello there sp \n Hey hey sp How are you? sp
[download]

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broquaint