Regex to find and expression and insert into the middle

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

Use

bsections*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 = '/bsections*d+(?:.d+)*:/';
$str = 'section 1.1: Disability ........';
$subst = '<hr/>$0';
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;