If you really want to use a regular expression for this, it would be something like /^.*?=(.*)$/. After that regex, $1 would hold your ID.

A regular expression isn't the right tool for that. Regular expressions can be expensive in terms of efficiency. So, you should favor other methods when possible.

For those strings use split ...

use strict;
use warnings;
my @strings = (
    "http://mysite/bbsui.jsp?id=dxpwd",
    "http://mysite/bbsui.jsp?id=dxpsf",
    "http://mysite/bbsui.jsp?id=sfpwd",
    "http://mysite/bbsui.jsp?id=ds35e",
    "http://mysite/bbsui.jsp?id=124536"
);

for ( @strings ) {
    my ($base, $id) = split( /=/, $_, 2 );
    print "Id: $id\n";
}
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