根据周围的字符/数值替换不同的值：gsub R.

Question

I have sample vector as follows:

vec1 <- c('3/4 in. of water', 'Indoor/Outdoor applications')

Now, I would like to replace '/' as 'by' if values surrounding '/' are numbers and as ' ' if values surrounding '/' are strings

I know the regex to match is either:

gsub('\\d+\\/\\d+', 'by', vec1)
gsub('\\w+\\/\\w+', 'by', vec1)

However, they give the following results:

"by in. of water"
"by in. of water" "by applications"

I would like the result as follows:

'3 by 4 in. of water',  'Indoor Outdoor applications'

Appreciate any inputs on how I can get these results.

Thanks!

Answer 1

You can use a PCRE regex patterns for that. The (?<=\\\\d)/(?=\\\\d) matches forward slashes thst are enclosed with digits. The /(?!\\\\d)|(?<!\\\\d)/ matches either a slash that has no digit on the right, or a slash with no digit in the left.

Here is a solution with gsub :

> vec1 <- c('3/4 in. of water', 'Indoor/Outdoor applications') 
> gsub("/(?!\\d)|(?<!\\d)/", " ", gsub("(?<=\\d)/(?=\\d)", " by ", vec1, perl=T), perl=T)
[1] "3 by 4 in. of water"         "Indoor Outdoor applications"

You can use mgsub from the qdap package and define the pattern and replacement character vectors.

See sample code:

> library(qdap)
> vec1 <- c('3/4 in. of water', 'Indoor/Outdoor applications') 
> repl <- c(' by ', ' ') 
> patt <- c('(?<=\\d)/(?=\\d)', '/(?!\\d)|(?<!\\d)/')
> mgsub(patt, repl, vec1, fixed=FALSE, perl=T)
## [1] "3 by 4 in. of water" "Indoor Outdoor applications"

Answer 2

gsub('(\\\\d+)\\\\/(\\\\d+)', '\\\\1 by \\\\2', vec1)

gsub('(\\\\w+)\\\\/(\\\\w+)', '\\\\1 \\\\2', vec1)

The parentheses around the \\\\d+ and \\\\w+ mean, "capture whatever is in these parentheses so we can use it later".

The first set of parentheses can be used later by referring to \\1 , the second set by \\2 , etc (and since we need to escape it here, make that \\\\1 and \\\\2 ).

When we go on to say what we want to replace our match with, this is when we can refer to those "captured" portions of the pattern/match, as you can see us doing in the pattern at the top.