Situation
I want to use preg_replace() to add a digit '8' after each of [aeiou].
Example
from
abcdefghij
to
a8bcde8fghi8j
Question
How should I write the replacement string?
// input string
$in = 'abcdefghij';
// this obviously won't work ----------↓
$out = preg_replace( '/([aeiou])/', '\18', $in);
This is just an example, so suggesting str_replace() is not a valid answer.
I want to know how to have number after backreference in the replacement string.
The solution is to wrap the backreference in ${}.
$out = preg_replace( '/([aeiou])/', '${1}8', $in);
which will output a8bcde8fghi8j
See the manual on this special case with backreferences.
"${1}8" does not work, because double quoted strings are parsed/interpreted in php and therefore the braces "disappear" disappear before the string is passed to the function.You can do this:
$out = preg_replace('/([aeiou])/', '${1}' . '8', $in);
Here is a relevant quote from the docs regarding backreference:
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.
