PHP中的正则表达式将用户名更改为漂亮的链接

Question

I am using a PHP regular expression to change usernames to pretty links in text.
Firstly, It looks for an @ sign,
Secondly, It checks if the @ sign has a user id touching it, E.G. @User10,
Thirdly, It makes sure that the user id is between 1 and 20 characters,
Fourthly it makes the links pretty.

This is my Regex

/(?<=^|\s|[([])\@([a-zA-Z0-9_]{1,20})/

This regex works fine for these text examples

@User1 This is a test string @User2
@User3

But it does not work in these text examples

<br>@User1 Hello John@User2

All of these Users should be matched but they are not being matched if a string is touching the @ sign before it.

I have an online regex example set here. http://regex101.com/r/zK9kC4

Answer 1

I tried this expression on http://gskinner.com/RegExr/

/@([a-zA-Z0-9_]{1,20})/

It matches all criteria you have given for the User-Id's.