R 中以负回溯开头的正则表达式可选字符

Question

Suppose I have a set of strings:

test <- c('MTB', 'NOT MTB', 'TB', 'NOT TB')

I want to write a regular expression to match either 'TB' or 'MTB' (eg, the expression "M?TB") strictly when this FAILS to be preceeded by the phrase "NOT " (space included).

My intended result, therefore, is

TRUE FALSE TRUE FALSE

So far I have tried a couple of variations of

grepl("(?<!NOT )M?TB", test, perl = T)

TRUE TRUE TRUE FALSE

Unsuccessfully. As you can see, the phrase 'NOT MTB' meets the criteria for my regular expression.

It seems like including the optional character "M?" seems to make R think that the negative lookbehind is also optional. I have been looking into using parentheses to group the patterns, such as

grepl("(?<!NOT )(M?TB)")

TRUE TRUE TRUE FALSE

Which also fails to exclude the phrase 'NOT MTB'. Admittedly, I am unclear on how parentheses work in regex or eeven what "grouping" means in this context. I have had trouble finding a question related to how to group, require, and "optionalize" different parts of a regex so that I can match a phrase beginning with an optional character and preceeded by a negative lookback . What is the proper way to write an expression like this?

Answer 1

We could use the start ( ^ ) and end ( $ ) to match only those words

grepl("^M?TB$", test)
#[1]  TRUE FALSE  TRUE FALSE

---

If there are other strings as @Wiktor Stribiżew mentioned in the comments, then one option would be

test1 <- c(test, "THIS MTB")
!grepl("\\bNOT M?TB\\b", test1) & grepl("\\bM?TB\\b", test1)
#[1]  TRUE FALSE  TRUE FALSE  TRUE

Answer 2

test = c("MTB", "NOT MTB", "TB", "NOT TB", "THIS TB", "THIS NOT TB")

grepl("\\b(?<!NOT\\s)M?TB\\b",test,perl = TRUE)

[1]  TRUE FALSE  TRUE FALSE  TRUE FALSE

Answer 3

There is some question on what the question intends but here is some code to try depending on what is wanted.

Added: Poster clarified that #2 and #3 are along the lines looked for.

1) This can be done without regular expressions like this:

test %in% c("TB", "MTB")
## [1]  TRUE FALSE  TRUE FALSE

2) If the problem is not about exact matches then return matches to M?TB which do not also match NOT M?TB:

grepl("M?TB", test) & !grepl("NOT M?TB",test)
## [1]  TRUE FALSE  TRUE FALSE

3) Another alternative is to replace NOT M?TB with X and then grepl on M?TB:

grepl("M?TB", sub("NOT M?TB", "X", test))
## [1]  TRUE FALSE  TRUE FALSE