x <- 'aabb.ccdd'
x
## [1] "aabb.ccdd"
sub('bb.*', '', x)
## [1] "aa"
sub('.*', '', x)
## [1] ""
sub('.*bb', '', x)
## [1] ".ccdd"
sub('\\..*', '', x)
## [1] "aabb"
sub('.*\\.', '', x)
## [1] "ccdd"