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)
> [         


        

Answer 1

you need double-escaping to pass escape to regex:

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

Answer 2

"\<" 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 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