options other than regex

Evan,

are you really looking at numbers, or just at character strings (that,
in your case, happen to be numbers)?  if just characters, this rather
odd combination of strsplit() and Reduce() might do the trick:
----

x <- '10110111'
print(x)

[1] "10110111"

y <- Reduce(function (x,y) { paste(x, y, sep=".") }, unlist(strsplit(x, "")), init="", right=TRUE)
print(y)

[1] "1.0.1.1.0.1.1.1."
----

cheers.
.