escaping pipe ("|") in a regex

Question

I have a need to split on words and end marks (punctuation of certain types). Oddly pipe ("|") can count as an end mark. I have code that words on end marks until I try to add the pipe. Adding the pipe makes the strsplit every character. Escaping it causes and error. How can I include the pipe int he regular expression?

x <- "I like the dog|."

strsplit(x, "[[:space:]]|(?=[.!?*-])", perl=TRUE)
#[[1]]
#[1] "I"    "like" "the"  "dog|" "."   

strsplit(x, "[[:space:]]|(?=[.!?*-\|])", perl=TRUE)
#Error: '\|' is an unrecognized escape in character string starting "[[:space:]]|(?=[.!?*-\|"

The outcome I'd like:

#[[1]]
#[1] "I"    "like" "the"  "dog"  "|"  "."  #pipe is an element

I am always hesitant to put regex tags on R regex questions because you get regexers from other languages and though the answers are similar they don't overlap. — Tyler Rinker, Oct 17 '12 at 19:01

score 19 · Accepted Answer · edited Oct 17 '12 at 20:34

19

One way to solve this is to use the \Q...\E notation to remove the special meaning of any of the characters in .... As it says in ?regex:

If you want to remove the special meaning from a sequence of characters, you can do so by putting them between ‘\Q’ and ‘\E’. This is different from Perl in that ‘$’ and ‘@’ are handled as literals in ‘\Q...\E’ sequences in PCRE, whereas in Perl, ‘$’ and ‘@’ cause variable interpolation.

For example:

> strsplit(x, "[[:space:]]|(?=[\\Q.!?*-|\\E])", perl=TRUE)
[[1]]
[1] "I"    "like" "the"  "dog"  "|"    "."

edited Oct 17 '12 at 20:34

Blue Magister

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answered Oct 17 '12 at 18:44

Joshua Ulrich

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Interesting. I definitely need to read up on regex some more. +1 – A5C1D2H2I1M1N2O1R2T1 Oct 17 '12 at 18:47
@Joshua, mrdwab gave a great answer and this is even more thorough. I never even heard of the `\\Q ... \\E` thing. very useful information. – Tyler Rinker Oct 17 '12 at 18:49
@TylerRinker: I'd never heard of it either, until I read the documentation after seeing your question. ;-) – Joshua Ulrich Oct 17 '12 at 18:51
@Joshua, you're going to make a statement like that and not provide the link :P – Tyler Rinker Oct 17 '12 at 18:54
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@TylerRinker: As I said in my answer, it's in `?regex`, see the *Perl-like Regular Expressions* section. – Joshua Ulrich Oct 17 '12 at 18:57
There it is. Thank you +1 for comment. – Tyler Rinker Oct 17 '12 at 19:00

score 12 · Answer 2 · edited May 23 '17 at 10:28

12

The problem is actually your hyphen, which should come either first or last:

strsplit(x, "[[:space:]]|(?=[|.!?*-])", perl=TRUE)
strsplit(x, "[[:space:]]|(?=[.|!?*-])", perl=TRUE)
strsplit(x, "[[:space:]]|(?=[.!|?*-])", perl=TRUE)
strsplit(x, "[[:space:]]|(?=[-|.!?*])", perl=TRUE)

and so on should all give you the output you are looking for.

You can also escape the hyphen if you prefer, but remember to use two backslashes!

strsplit(x, "[[:space:]]|(?=[.!?*\\-|])", perl=TRUE)

edited May 23 '17 at 10:28

Community

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answered Oct 17 '12 at 18:35

A5C1D2H2I1M1N2O1R2T1

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Both excellent answers, josh gave even more detail. Thank you for your answer +1 – Tyler Rinker Oct 17 '12 at 18:50

escaping pipe ("|") in a regex

2 Answers2