Question

found this regex:

insert " " every 10 characters:  
$text =  preg_replace("|(.{10})|u", "\${1}"." ", $text);

can you, please, explain what \${1} means. Why using \ and what curly brackets means?

Answer 1

Quoting some portions of the manual page of preg_replace :

replacement may contain references of
the form \\n or $n, with the latter
form being the preferred one.

You are, apparently, in the second case : $n

And, later :

When working with a replacement
pattern where a backreference is
immediately followed by another number
(i.e.: placing a literal number
immediately after a matched pattern),
you cannot use the familiar \\1
notation for your backreference.

\\11, for example, would confuse
preg_replace() since it does not know
whether you want the \\1 backreference
followed by a literal 1, or the \\11
backreference followed by nothing.

In this case the solution is to
use \${1}1. This creates an isolated
$1 backreference, leaving the 1 as a
literal.

Here, you don’t have anything after what would be $1 — but I suppose it cannot hurt to use the \${1} notation : I find it makes code easier to read, having those {} ; and it makes sure you won’t forget to add them the day they are needed.