PHP core technology and best practice of the regular expression of the reverse reference ____php

PHP core technology and best practices in reverse reference to regular expressions

A reverse reference is used to repeat the search for text that matches a previous grouping. For example: "\1" represents grouped 1 matching text:

\b (\w+) \b\s+\1\b

To reverse reference to the contents of a group capture, you can use the previous example of "\k<word>":

\b (? <word>\w+) \b\s+\k<word>\b

"Example": Ubb Tag Code

UBB tags are used to replace HTML in forums and message boards, to achieve some simple HTML effects, while preventing the misuse of HTML for security issues.

The final UBB tag will still be parsed into HTML code to allow the browser to recognize the URL tag as an example.

"[Url]1.gif[/url]"

? PHP
       $str = ' [Url]1.gif[/url][url]2.gif[/url] [Url]3.gif[/url] ';
       $s = Preg_replace ("#\[url\] (? <word>\d\.gif) \[\/url\]#", ";
       Var_dump ($s);
? >