Why the argument that preg_replace_callback passes to the callback function is an array

Question

This might be a stupid question but why preg_replace_callback puts every match in a single-element array and passes it to the callback function so I have to use $matches[0] to process a match, why doesn't it pass matches to the callback function as strings?

Answer 1

Because you can have capturing groups, and each element in the array would be what the capturing group captured. The element at index 0 is always the entire match.

For example, given a regex that matches MM/DD/YYYY dates, you might put each segment of the date in its own capturing group, perhaps something naive like this:

(\d{2})/(\d{2})/(\d{4})

Then, you'd have a matches array similar to:

[0] - MM/DD/YYYY
[1] - MM
[2] - DD
[3] - YYYY