w3schools - How can I get at the matches when using preg_replace in PHP?
From the preg_replace documentation on php.net:
replacement may contain references of the form \n or (since PHP 4.0.4) $n, with the latter form being the preferred one. Every such reference will be replaced by the text captured by the n'th parenthesized pattern.
See Flubba's example.
I am trying to grab the capital letters of a couple of words and wrap them in span tags. I am using preg_replace for extract and wrapping purposes, but it's not outputting anything.
preg_replace("/[A-Z]/", "<span class=\"initial\">$1</span>", $str)
Use parentheses around your desired capture.
will also match the entire matched expression without doing an explicit capture using parenthesis.
preg_replace("/[A-Z]/", "<span class=\"initial\">\\0</span>", $str)
As always, you can go to php.net/preg_replace or php.net/<whatever search term> to search the documentation quickly. Quoth the documentation:
\0 or $0 refers to the text matched by the whole pattern.