Regex to find and expression and insert into the middle - php

Question

I'll be brief as I can. I'm trying to use preg_replace's regex to find a digit, but I want to non destructively edit the string.
an example: (albeit this is an approximation due to data protection)
$subject_string = 'section 1.1: Disability ........' ;
$outcome = preg_replace( '/$section[\d.\d]+/' , '\<hr/\>' , $subject_string );
// $outcome will be: "\<hr/\>section 1.1: Disability ........"
Any help would be gratefully received

Answer 1

Use
\bsection\s*\d+(?:\.\d+)*:
Replace with <hr/>$0. See regex proof.
EXPLANATION
--------------------------------------------------------------------------------
\b the boundary between a word char (\w) and
something that is not a word char
--------------------------------------------------------------------------------
section 'section'
--------------------------------------------------------------------------------
\s* whitespace (\n, \r, \t, \f, and " ") (0 or
more times (matching the most amount
possible))
--------------------------------------------------------------------------------
\d+ digits (0-9) (1 or more times (matching
the most amount possible))
--------------------------------------------------------------------------------
(?: group, but do not capture (0 or more times
(matching the most amount possible)):
--------------------------------------------------------------------------------
\. '.'
--------------------------------------------------------------------------------
\d+ digits (0-9) (1 or more times (matching
the most amount possible))
--------------------------------------------------------------------------------
)* end of grouping
--------------------------------------------------------------------------------
: ':'
Code snippet:
$re = '/\bsection\s*\d+(?:\.\d+)*:/';
$str = 'section 1.1: Disability ........';
$subst = '<hr/>$0';
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;