Fix how kable_mark() measures column width when there are hyperlinks

[jacobbien]

This is intended to address #2147.

Recall the code example there:

very_long_url <- paste(c("a", rep("very", 20), "long-url"), collapse = "-")
tbl <- data.frame(
  flavor = sprintf("[Mint chip](%s)", very_long_url),
  description = "Of all the many flavors of ice cream, this is my favorite."
  )
knitr::kable(tbl, format = "pipe")

It now generates a different output:

|flavor    |description                                                |
|:---------|:----------------------------------------------------------|
|[Mint chip](a-very-very-very-very-very-very-very-very-very-very-very-very-very-very-very-very-very-very-very-very-long-url)|Of all the many flavors of ice cream, this is my favorite. |

And here's what the table in the html file now looks like:

Recall that before it looked like

[CLAassistant]

All committers have signed the CLA.

[yihui]

Thanks a lot!

[yihui]

Could you also use zero-width positive/negative lookbehind/lookahead assertions (see ?regex if you are not familiar with them) to avoid treating `[text](url)` as a URL?

[jacobbien]

Below are two examples of using positive lookbehind/lookahead for matching and replacing. Perhaps I'm missing something, but it seems like the fact that it excludes the brackets and parentheses from the match actually makes it harder to remove them. Both examples have the same regex, which is to look for [x]( where x is 0 or more non-] characters.

1. Matching

stringr::str_match_all("[label1](url1) and [label2](url2).","(?<=\\[)[^\\]]*(?=\\]\\()")

[[1]]
     [,1]    
[1,] "label1"
[2,] "label2"

2. Replacing

stringr::str_replace_all("[label1](url1) and [label2](url2).","(?<=\\[)[^\\]]*(?=\\]\\()", "MATCHED")

"[MATCHED](url1) and [MATCHED](url2)."

But I'm new to zero-width lookbehind/lookahead, so perhaps I'm missing something. Thanks so much for writing knitr, by the way -- it's such a valuable tool!

[yihui]

Sorry I guess I was not clear last time. I meant we should exclude the case of []() surrounded by backticks (i.e., (?<!`) and (?!`)).

[jacobbien]

Ah, I see! Thanks for explaining that. I have updated the PR accordingly.

[yihui]

Perfect. Thanks!