Regular Expressions

<pre[^>]*>(.*)(?:<a[^>]*>(.*)</a>(.*))*</pre>

\b\w{3,15}\b

(?!^[a-zA-Z0-9\.\-_]{1,2}$)(?!^[a-zA-Z0-9\.\-_]{16,}$)(?!^[0-9]{3,15}$)^(?:[a-zA-Z0-9](?:\.|_|-)?)+[a-zA-Z0-9]$

1. Zero-width lookahead
   This essentially means that this matching expression will not consume any characters. So when this expression has matched any following non zero.width expression will start of at the same place in the string as the zero-width regular expression did.
2. Negative
   This signifies that all following expressions can only match, if this expression did not produce any matches.

 (?!^[a-zA-Z0-9\.\-_]{1,2}$) // This expression will assert that a username with characters a-z A-Z 0-9 . _ and - are not less than three characters long
 (?!^[a-zA-Z0-9\.\-_]{16,}$) // This expression will assert that a username with characters a-z A-Z 0-9 . _ and - are no more than 15 characters long
 (?!^[0-9]{3,15}$)           // This expression will assert that a username will not be made up of only digits [0-9]
 ^(?:[a-zA-Z0-9](?:\.|_|-)?)+[a-zA-Z0-9]$

    ^                // Start match at the beginning to the string
    (?:[a-zA-Z0-9]   // Make sure that a username will not start with a '.' a '-' or a '_', but only alphanumeric characters
        (?:\.|_|-)?  // After an initial alpha numeric a '.' a '-' or a '_' may follow, but not two in a row ...
    )+               // ... so we repeat this pattern once or many (+). Drawback: ._ .- _- etc. are also not allowed, which I think we can live with here.
    [a-zA-Z0-9]      // Finally we make sure that there is an alphanumeric character in the last position ...
    $                // ... exactly at the end of the string.