Split character vector at math comparisons signs in R

Question

I would like to split expression with mathematical comparisons, e.g.

unlist(strsplit("var<3", "(?=[=<>])", perl = TRUE))
unlist(strsplit("var==5", "(?=[=<>])", perl = TRUE))
unlist(strsplit("var>2", "(?=[=<>])", perl = TRUE))

The results are:

[1] "var" "<"   "3"  
[1] "var" "="   "="   "5"  
[1] "var" ">"   "2"  

For the 2nd example above, I would like to get [1] "var" "==" "5", so the two = should be returned as a single element. How do I need to change my regular expression to achieve this? (I already tried grouping and quantifiers for "==", but nothing worked - regular expressions are not my friends...)

Answer 1

You may use a PCRE regex to match the substrings you need:

==|[<>]|(?:(?!==)[^<>])+

To also support !=, modify it as

[!=]=|[<>]|(?:(?![=!]=)[^<>])+

See the regex demo.

Details:

• == - 2 = signs
• | - or
• [<>] - a < or >
• | - or
• (?:(?!==)[^<>])+ - 1 or more chars other than < and > ([^<>]) that do not start a == char sequence (a tempered greedy token).

NOTE: This is easily expandable by adding more alternatives and adjusting the tempered greedy token.

R test:

> text <- "Text1==text2<text3><More here"
> res <- regmatches(text, gregexpr("==|[<>]|(?:(?!==)[^<>])+", text, perl=TRUE))
> res
[[1]]
[1] "Text1"     "=="        "text2"     "<"         "text3"     ">"        
[7] "<"         "More here"

Answer 2

Expanding from my idea in comments, just for the formatting:

tests=c("var==5","var<3","var.name>5")
regmatches(tests,regexec("([a-zA-Z0-9_.]+)(\\W+)([a-zA-Z0-9_.]+)",tests))

\w is [a-zA-Z0-9_] and \W it's opposite [^a-zA-Z0-9_], I expanded it after comment to include . in the character class, and as R doesn't support \w in character class in base regex (need to use perl=TRUE).

So the regex search for a least 1 of \w and ., then a least 1 not in \w (to match operators) and then a least 1 of \w and dot.

Each step is captured, and this give:

[[1]]
[1] "var==5" "var"    "=="     "5"     

[[2]]
[1] "var<3" "var"   "<"     "3"    

[[3]]
[1] "var.name>5" "var.name"   ">"          "5"       

you may add * between each capture group if your entries could have space around the operator, if not the operator capture will get them.