What does u mean in regex?

I came across this code and I'm at loss as to what u means :

$todecode =~ s{
   %u([Dd][89a-bA-B][0-9a-fA-F]{2}) # hi
          %u([Dd][c-fC-开发者_Go百科F][0-9a-fA-F]{2})   # lo
        }{
     utf8_chr(
       0x10000_
       + (hex($1) - 0xD800) * 0x400_
       + (hex($2) - 0xDC00)
      )
        }gex;

It's the letter between t and v. (It's matching a literal u.)

It looks like somebody has some text with UTF-16 surrogate pairs written out as %uD800%uDC00, and they're passing the decoded codepoint to the utf8_chr function, and substituting the result of that.