R Regular expression for string containing full stops

Question

I have a bunch of strings, some of which end with ..t.. I am trying to find a regular expression to match these strings but dealing with the full stops is giving me a headache!

I have tried

grep('^.+(..t.)$', myStrings)

but this also matches strings such as w...gate. I think I am dealing with the full stops incorrectly. Any help at all appreciated.

Note: I am using grep within R.

Answer 1

Since you are only checking if the end of the string ends with ..t., you can eliminate ^.+ in your pattern.

The dot . in regular expression syntax is a character of special meaning which matches any character except a newline sequence. To match a literal dot or any other character of special meaning you need to escape \\ it.

> x <- c('foo..t.', 'w...gate', 'bar..t.foo', 'bar..t.')
> grep('\\.{2}t\\.$', x)
# [1] 1 4

Or place that character inside of a character class.

> x <- c('foo..t.', 'w...gate', 'bar..t.foo', 'bar..t.')
> grep('[.]{2}t[.]$', x)
# [1] 1 4

Note: I used the range operator \\.{2} to match two dots instead of escaping it twice \\.\\.

Answer 2

k, a little bit of better googling provided the answer;

 grep("^.+(\\.\\.t\\.)$", myStrings)

this works because we need to escape the point as \\. in R.