Complete word matching using grepl in R

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".

Answer 2

you need double-escaping to pass escape to regex:

> grepl("\\bis\\b",testLines)
[1] FALSE  TRUE

Answer 3

Very simplistically, match on a leading space:

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

There's a whole lot more than this to regular expressions, but essentially the pattern needs to be more specific. What you will need in more general cases is a huge topic. See ?regex

Other possibilities that will work for this example:

grepl(' is ',testLines)
[1] FALSE  TRUE
grepl('\\sis',testLines)
[1] FALSE  TRUE
grepl('\\sis\\s',testLines)
[1] FALSE  TRUE