PHP - add underscores before capital letters

Question

How can I replace a set of words that look like:

SomeText

to

Some_Text

?

Answer 1

$s1 = "ThisIsATest";
$s2 = preg_replace("/(?<=[a-zA-Z])(?=[A-Z])/", "_", $s1);

echo $s2;  //  "This_Is_A_Test"

Explanation:

The regex uses two look-around assertions (one look-behind and one look-ahead) to find spots in the string where an underscore should be inserted.

(?<=[a-zA-Z])   # a position that is preceded by an ASCII letter
(?=[A-Z])       # a position that is followed by an uppercase ASCII letter

The first assertion makes sure that no underscore is inserted at the start of the string.