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rregexregular-language

Regular expression matching on comma bounded by nonwhite space

I am trying to replace commas bounded by nonwhite space with a white space, while keeping other commas untouched (in R).

Imagine I have:

j<-"Abc,Abc, and c"

and I want: 

"Abc Abc, and c"

This almost works: 

gsub("[^ ],[^ ]"," " ,j)

But it removes the characters either side of the commas to give:

"Ab bc, and c"
Solution

  • You may use a PCRE regex with a negative lookbehind and lookahead:

    j <- "Abc,Abc, and c"
gsub("(?<!\\s),(?!\\s)", " ", j, perl = TRUE)
## => [1] "Abc Abc, and c"

    See the regex demo

    Details:

    • (?<!\\s) - there cannot be a whitespace right before a ,
    • , - a literal ,
    • (?!\\s) - there cannot be a whitespace right after a ,

    An alternative solution is to match a , that is enclosed with word boundaries:

    j <- "Abc,Abc, and c"
gsub("\\b,\\b", " ", j)
## => [1] "Abc Abc, and c"

    See another R demo.