[R] Need some help with regular expression

[Bert Gunter]

Although others may respond, I think you will do much better studying
?regexp, which will answer all your questions. I believe the effort
you will make figuring it out will pay dividends for your future
R/regular expression usage that you cannot gain from my direct
explanation.

Good luck.

Best,
Bert
Bert Gunter

"The trouble with having an open mind is that people keep coming along
and sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )


On Sun, Nov 20, 2016 at 6:40 PM, Steven Nagy <nstefi at gmail.com> wrote:
> Thanks a lot Bert. That's amazing. I am very new to both R and regular
> expressions. I don't really understand the regular expression that you used
> below.
> And looks like I don't even need any special library, like the "gsubfn" for
> the strapply function.
> I was trying to use the regexr.com website to analyze your regular
> expression, but it doesn't seem to match any text there.
> Can you explain me the regular expression that you used?
> ".*: *([[:alnum:]]* *-> *STU|STU *-> *[[:alnum:]]*).*"
> So the dot in the front means any character and the star after that means
> that it can repeat 0 or more times, right?
> Then followed by a colon character ":" and a space, and what is the next
> star after that? It means that the sequence before that again can repeat 0
> or more times?
> And what are the double square brackets?
> Is ":alnum:" specific to R? I don't think "regexr.com" understands that. Or
> maybe that site is for regular expressions in Javascript, and the syntax is
> different in R?
>
> Thank you,
> Steven
>
> -----Original Message-----
> From: Bert Gunter [mailto:bgunter.4567 at gmail.com]
> Sent: Sunday, November 20, 2016 2:15 PM
> To: Steven Nagy <nstefi at gmail.com>
> Cc: R-help <r-help at r-project.org>
> Subject: Re: [R] Need some help with regular expression
>
> If I understand you correctly, I think you are making it more complex than
> necessary. Using your example (thanks!!), the following should get you
> started:
>
>
>> x<- c("Name.MEMBER_TYPE: NMA -> STU ; CATEGORY:  -> 1 ; CITY:
>> MISSISSAUGA -> Mississauga ; ZIP: L5N1H9 -> L5N 1H9 ; COUNTRY: CAN ->
>> ; MEMBER_STATUS:  -> N", "Name.MEMBER_TYPE: STU -> REG ; CATEGORY: 1
>> ->","Name.MEMBER_TYPE: -> STU")
>>
>> x
> [1] "Name.MEMBER_TYPE: NMA -> STU ; CATEGORY:  -> 1 ; CITY:
> MISSISSAUGA -> Mississauga ; ZIP: L5N1H9 -> L5N 1H9 ; COUNTRY: CAN -> ;
> MEMBER_STATUS:  -> N"
>
> [2] "Name.MEMBER_TYPE: STU -> REG ; CATEGORY: 1 ->"
> [3] "Name.MEMBER_TYPE: -> STU"
>>
>> sub(".*: *([[:alnum:]]* *-> *STU|STU *-> *[[:alnum:]]*).*","\\1",x)
> [1] "NMA -> STU" "STU -> REG" "-> STU"
>
>
> I am sure that you can get things to the form you desire in one go with some
> fiddling of the above, but it was easier for me to write the regex to pick
> out the pieces you wanted and leave the rest to you.
> Others may have slicker ways to do it, of course.
>
> HTH
>
> Cheers,
> Bert
>
>
> Bert Gunter
>
> "The trouble with having an open mind is that people keep coming along and
> sticking things into it."
> -- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )
>
>
> On Sat, Nov 19, 2016 at 8:06 PM, Steven Nagy <nstefi at gmail.com> wrote:
>> I tried out a regular expression on this website:
>>
>> http://regexr.com/3en1m
>>
>>
>>
>> So the input text is:
>>
>> "Name.MEMBER_TYPE:  -> STU"
>>
>>
>>
>> The regular expression is: ((?:\w+|\s) -> STU|STU -> (?:\w+|\s))
>>
>> And it returns:
>>
>> "  -> STU"
>>
>>
>>
>> but when I use in R, it doesn't return the same result:
>>
>> strapply(c, "((?:\\w+|\\s) -> STU|STU -> (?:\\w+|\\s))", c, backref =
>> -1, perl = TRUE)
>>
>> returns:
>> "Name.MEMBER_TYPE: -> STU"
>>
>>
>>
>>
>>
>> Here is what I was trying to do:
>>
>>
>>
>> I need to extract some values from a log table, and I created a
>> regular expression that helps me with that.
>>
>> The log table has cells with values like:
>>
>> a = "Name.MEMBER_TYPE: NMA -> STU ; CATEGORY:  -> 1 ; CITY:
>> MISSISSAUGA -> Mississauga ; ZIP: L5N1H9 -> L5N 1H9 ; COUNTRY: CAN ->
>> ; MEMBER_STATUS:  -> N"
>>
>> or
>> b = "Name.MEMBER_TYPE: STU -> REG ; CATEGORY: 1 ->"
>>
>> so I needed to extract the values that a STU member type is changing
>> from and to, so I needed NMA, STU in the 1st case or STU, REG in the 2nd
> case.
>>
>> I came up with this expression which worked in both cases:
>>
>> strapply(strapply(a, "(\\w+ -> STU|STU -> \\w+)", c, backref = -1,
>> perl = TRUE), "(\\w+) -> (\\w+)", c, backref = -2, perl = TRUE)
>>
>>
>>
>> But I had a 3rd case when the source member type was blank:
>>
>> c = "Name.MEMBER_TYPE: -> STU"
>>
>> and in that case it returned an error:
>>
>> strapply(strapply(c, "(\\w+ -> STU|STU -> \\w+)", c, backref = -1,
>> perl = TRUE), "(\\w+) -> (\\w+)", c, backref = -2, perl = TRUE)
>>
>> Error: is.character(x) is not TRUE
>>
>>
>>
>> I found that the error is because this returns NULL:
>>
>> strapply(c, "(\\w+ -> STU|STU -> \\w+)", c, backref = -1, perl = TRUE)
>>
>>
>>
>>
>>
>> So I tried to modify the regular expression to match any word or blank
>> space:
>>
>> strapply(c, "((?:\\w+|\\s) -> STU|STU -> (?:\\w+|\\s))", c, backref =
>> -1, perl = TRUE)
>>
>>
>>
>> but this returned me the whole value of "c":
>>
>> "Name.MEMBER_TYPE:  -> STU"
>>
>> and I only needed "  -> STU" as it shows on the website regxr.com
>>
>>
>>
>> Is the result wrong on the regxr.com website or strapply returns the
>> wrong result?
>>
>>
>>
>> Thanks,
>>
>> Steven
>>
>>
>>         [[alternative HTML version deleted]]
>>
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