Regular expression: ends with a pattern followed by a/or string

Help regular expression: End with a pattern followed by a/or string
I want a match pattern followed by/or the end of the string.

Requirements are as follows: two-segment domain name modified to three (increase www.)

Original string: http://a.com or http://a.com/or http://www.a.com/
To be replaced by: http://www.a.com

Http://k.a.com cannot be replaced by http://www.k.a.com

I wrote:
Preg_replace ('/(\/\/) ([^.\/]+\.[ ^.\/]+$)/I ', ' $1www.$2 ', ' http://weibo.com ');
Preg_replace ('/(\/\/) ([^.\/]+\.[ ^.\/]+\/)/I ', ' $1www.$2 ', ' http://weibo.com/');
Requires two substitutions, can you put the \/and $ together, similar to [A2]
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Preg_replace ('/(\/\/) ([^.\/]+\.[ ^.\/]+(?:\ /
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$))/I ', ' $1www.$2 ', ' http://weibo.com ');

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Give it a try

echo preg_replace ('/(\/\/) ([^.\/]+\.[ ^.\/]+)($
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\/) (. +)?/ie ', ' "$". " www. "." ". ("$4"?) "/"." $4 ":" ") ', ' http://weibo.com/aa ');

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