You can use gregexpr
for example to find the desired pattern and regmatches
to extract it:
txt = c('Bleh Blah #a #b','Blah Bleh #a')
regmatches(txt,gregexpr('#[a-z]',txt)) ## I assume a tag is # followed by lower letter
[[1]]
[1] "#a" "#b"
[[2]]
[1] "#a"
using alexis example, you write something like this:
DF$tag <- regmatches(DF$Post,gregexpr('#[a-z]',DF$Post)
edit in case tag is someething like #hi (more than one letter):
txt = c('Bleh Blah #hi allo #b','Blah Bleh #a')
regmatches(txt,gregexpr('#[a-z]+',txt))
[1]]
[1] "#hi" "#b"
[[2]]
[1] "#a"