Identifying common prefixes from a vector of words, and delete those prefixes

For example, c("dog.is.an.animal", "cat.is.an.animal", "rat.is.an.
animal"). How can I identify the common prefix is ".is.an.animal" 
and delete it to give c("dog", "cat", "rat") ?

foo <- c("dog.is.an.animal", "cat.is.an.animal", "rat.is.an.animal")
sub(".is.an.animal", "", foo)

Being pedantic, ".is.an.animal" is a suffix not a prefix since it comes 
afterwards.

Regards,
Richie.

Mathematical Sciences Unit
HSL


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