string <- 'sp|Q2UVX4|CO3_BOVIN Complement C3 OS=Bos taurus OX=9913 GN=C3 PE=1 SV=2'
library(stringr)
str_match(string, '[a-z]{2}\\|[|\\w]*[A-Z]\\s+(.*?)\\s+OS=')[,2]
#[1] "Complement C3"
c3RyaW5nIDwtICdzcHxRMlVWWDR8Q08zX0JPVklOIENvbXBsZW1lbnQgQzMgT1M9Qm9zIHRhdXJ1cyBPWD05OTEzIEdOPUMzIFBFPTEgU1Y9MicKbGlicmFyeShzdHJpbmdyKQpzdHJfbWF0Y2goc3RyaW5nLCAnW2Etel17Mn1cXHxbfFxcd10qW0EtWl1cXHMrKC4qPylcXHMrT1M9JylbLDJdCiNbMV0gIkNvbXBsZW1lbnQgQzMiCiA=