Question

Consider the following example:

> testLines <- c("I don't want to match this","This is what I want to match")
> grepl('is',testLines)
> [1] TRUE TRUE

What I want, though, is to only match ‘is’ when it stands alone as a single word. From reading a bit of perl documentation, it seemed that the way to do this is with \b, an anchor that can be used to identify what comes before and after the patter, i.e. \bword\b matches ‘word’ but not ‘sword’. So I tried the following example, with use of Perl syntax set to ‘TRUE’:

> grepl('\bis\b',testLines,perl=TRUE)
> [1] FALSE FALSE

The output I’m looking for is FALSE TRUE.

Answer 1

“\<” is another escape sequence for the beginning of a word, and “\>” is the end.
In R strings you need to double the backslashes, so:

> grepl("\\<is\\>", c("this", "who is it?", "is it?", "it is!", "iso"))
[1] FALSE  TRUE  TRUE  TRUE FALSE

Note that this matches “is!” but not “iso”.