Regular expression: extract string between two characters/strings

Question

I have a model formula (as string) and want to extract the value for a specific argument, id in my case. Now I have found a way that returns the string without the needed string value. I want exactly the opposite, I only want the string value that is missing in my result:

xx <- "gee(formula = breaks ~ tension, id = wool, data = warpbreaks)"
sub("(?=(id=|id =))([a-zA-Z].*)(?=,)", "\\1", xx, perl =T)
#> [1] "gee(formula = breaks ~ tension, id =, data = warpbreaks)"

wool is missing in the return value, but I only want to have wool as resulting string... Can anyone help me finding the correct regex pattern?

Answer 1

Instead of regex here, you could parse() the string and grab the id argument by name.

as.character(parse(text = xx)[[1]]$id)
# [1] "wool"

Answer 2

You may use

xx <- "gee(formula = breaks ~ tension, id = wool, data = warpbreaks)"
sub(".*\\bid\\s*=\\s*(\\w+).*", "\\1", xx)
## or, if the value extracted may contain any chars but commas
sub(".*\\bid\\s*=\\s*([^,]+).*", "\\1", xx)

See the R demo and the regex demo .

Details

• .* - any 0+ chars, as many as possible
• \\\\bid - a whole word id ( \\b is a word boundary)
• \\\\s*=\\\\s* - a = enclosed with 0+ whitespaces
• (\\\\w+) - Capturing group 1 ( \\\\1 in the replacement pattern refers to this value): one or more letters, digits or underscores (or [^,]+ matches 1+ chars other than a comma)
• .* - the rest of the string.

Other alternative solutions:

> xx <- "gee(formula = breaks ~ tension, id = wool, data = warpbreaks)"
> regmatches(xx, regexpr("\\bid\\s*=\\s*\\K[^,]+", xx, perl=TRUE))
[1] "wool"

The pattern matches id , = enclosed with 0+ whitespaces, then \\K omits the matched text and only 1+ chars other than , land in the match value.

Or, a capturing approach with stringr::str_match is also valid here:

> library(stringr)
> str_match(xx, "\\bid\\s*=\\s*([^,]+)")[,2]
[1] "wool"