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?

This will do it: sub(".*id ?= ?(.*?),.*", "\\1", xx). You need to match the whole string. — lmo
– lmo, Commented Feb 10, 2019 at 18:11

Rich Scriven · Accepted Answer · 2019-02-10 18:23:08Z

3

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"

answered Feb 10, 2019 at 18:23

Rich Scriven

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1 Comment

Daniel Over a year ago

Thanks, this is also an elegant solution!

Wiktor Stribiżew · Accepted Answer · 2019-02-10 18:23:14Z

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"

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