Match URL parameter Regular expression

Find a match URL parameter regular expression
For example
http://www.sina.com.cn/sports/?p= 3456

http://www.sina.com.cn/sports/?p= 38923

http://www.sina.com.cn/sports/basktball/?p= 878923

Now we need to match the parameters inside the p=, which is the black part.
The way I use it is,
^/? ([a-z/]+)/\?p= ([0-9]+)
Can match the previous parameters, but there is no way to match the "? =p" after the number of parameters, I would like to ask this section of the regular expression how to write it? Thank you!

------Solution--------------------
Preg_match ('/p= (\d+)/', $s, $m);
echo $m [1];
------Solution--------------------
2loou Good
------Solution--------------------
Preg_match ('/(<=\?p=) \d+/', $str, $match);
echo $match [0];
------Solution--------------------

(? <=p\=) \d+

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