x = ">P04259 SWISS-PROT:P04259 Tax_Id=9606 Gene_Symbol=KRT6B Keratin, type II cytoskeletal 6B"
gsub("^(?:>.*?(?:ENSEMBL:|SWISS-PROT:|TREMBL:))([A-Z0-9.-]+)(?:.*?Gene_Symbol=(\\S+)|\\s|;\\S+ |\\|\\S+)(.*?)(?:;>.*|$)", "\\1", x, perl = TRUE)
eCA9ICI+UDA0MjU5IFNXSVNTLVBST1Q6UDA0MjU5IFRheF9JZD05NjA2IEdlbmVfU3ltYm9sPUtSVDZCIEtlcmF0aW4sIHR5cGUgSUkgY3l0b3NrZWxldGFsIDZCIgpnc3ViKCJeKD86Pi4qPyg/OkVOU0VNQkw6fFNXSVNTLVBST1Q6fFRSRU1CTDopKShbQS1aMC05Li1dKykoPzouKj9HZW5lX1N5bWJvbD0oXFxTKyl8XFxzfDtcXFMrIHxcXHxcXFMrKSguKj8pKD86Oz4uKnwkKSIsICJcXDEiLCB4LCBwZXJsID0gVFJVRSkK