Dear Avi,
Punctuation marks are used in various NLP language models. Preserving
the "," is therefore useful in such scenarios and Regex are useful to
accomplish this (especially if you have sufficient experience with such
expressions).
I observed only an odd behaviour using strsplit: the example string is
constructed; but it is always wise to test a Regex expression against
various scenarios. It is usually hard to predict what special cases will
occur in a specific corpus.
strsplit("a bc,def, adef ,,gh", " |(?=,)|(?<=,)(?![ ])", perl=T)
# "a" "bc" "," "def" "," "" "adef" "," "," "gh"
stringi::stri_split("a bc,def, adef ,,gh", regex=" |(?=,)|(?<=,)(?![ ])")
# "a" "bc" "," "def" "," "adef" "" "," "," "gh"
stringi::stri_split("a bc,def, adef ,,gh", regex=" |(?<!
)(?=,)|(?<=,)(?![ ])")
# "a" "bc" "," "def" "," "adef" "," "," "gh"
# Expected:
# "a" "bc" "," "def" "," "adef" "," "," "gh"
# see 2nd instance of stringi::stri_split
Sincerely,
Leonard
On 5/5/2023 11:20 PM, avi.e.gr...@gmail.com wrote:
Leonard,
It can be helpful to spell out your intent in English or some of us have to go
back to the documentation to remember what some of the operators do.
Your text being searched seems to be an example of items between comas with an
optional space after some commas and in one case, nothing between commas.
So what is your goal for the example, and in general? You mention a bit
unclearly at the end some of what you expect and I think it would be clearer if
you also showed exactly the output you would want.
I saw some other replies that addressed what you wanted and am going to reply
in another direction.
Why do things the hard way using things like lookahead or look behind? Would
several steps get you the result way more clearly?
For the sake of argument, you either want what reading in a CSV file would
supply, or something else. Since you are not simply splitting on commas, it
sounds like something else. But what exactly else? Something as simple as this
on just a comma produces results including empty strings and embedded leading
or trailing spaces:
strsplit("a bc,def, adef ,,gh", ",")
[[1]]
[1] "a bc" "def" " adef " "" "gh"
That can of course be handled by, for example, trimming the result after
unlisting the odd way strsplit returns results:
library("stringr")
str_squish(unlist(strsplit("a bc,def, adef ,,gh", ",")))
[1] "a bc" "def" "adef" "" "gh"
Now do you want the empty string to be something else, such as an NA? That can
be done too with another step.
And a completely different variant can be used to read in your one-line CSV as
text using standard overkill tools:
read.table(text="a bc,def, adef ,,gh", sep=",")
V1 V2 V3 V4 V5
1 a bc def adef NA gh
The above is a vector of texts. But if you simply want to reassemble your
initial string cleaned up a bit, you can use paste to put back commas, as in a
variation of the earlier example:
paste(str_squish(unlist(strsplit("a bc,def, adef ,,gh", ","))), collapse=",")
[1] "a bc,def,adef,,gh"
So my question is whether using advanced methods is really necessary for your
case, or even particularly efficient. If efficiency matters, often, it is
better to use tools without regular expressions such as paste0() when they meet
your needs.
Of course, unless I know what you are actually trying to do, my remarks may be
not useful.
-----Original Message-----
From: R-help <r-help-boun...@r-project.org> On Behalf Of Leonard Mada via R-help
Sent: Thursday, May 4, 2023 5:00 PM
To: R-help Mailing List <r-help@r-project.org>
Subject: [R] Regex Split?
Dear R-Users,
I tried the following 3 Regex expressions in R 4.3:
strsplit("a bc,def, adef ,,gh", " |(?=,)|(?<=,)(?![ ])", perl=T)
# "a" "bc" "," "def" "," "" "adef" "," "," "gh"
strsplit("a bc,def, adef ,,gh", " |(?<! )(?=,)|(?<=,)(?![ ])", perl=T)
# "a" "bc" "," "def" "," "" "adef" "," "," "gh"
strsplit("a bc,def, adef ,,gh", " |(?<! )(?=,)|(?<=,)(?=[^ ])", perl=T)
# "a" "bc" "," "def" "," "" "adef" "," "," "gh"
Is this correct?
I feel that:
- none should return (after "def"): ",", "";
- the first one could also return "", "," (but probably not; not fully
sure about this);
Sincerely,
Leonard
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