This question already has an answer here:
How to use back reference with stringi package?
(1 answer)
Closed 6 years ago.
Most stringr functions are just wrappers around corresponding stringi functions. str_replace_all is one of those. Yet my code does not work with stri_replace_all, the corresponding stringi function.
I am writing a quick regex to convert (a subset of) camel case to spaced words.
I am quite puzzled as to why this works:
str <- "thisIsCamelCase aintIt"
stringr::str_replace_all(str,
pattern="(?<=[a-z])([A-Z])",
replacement=" \\1")
# "this Is Camel Case ain't It"
And this does not:
stri_replace_all(str,
regex="(?<=[a-z])([A-Z])",
replacement=" \\1")
# "this 1s 1amel 1ase ain't 1t"
If you look at the source for stringr::str_replace_all you'll see that it calls fix_replacement(replacement) to convert the \\# capture group references to $#. But the help on stringi:: stri_replace_all also clearly shows that you use $1, $2, etc for the capture groups.
str <- "thisIsCamelCase aintIt"
stri_replace_all(str, regex="(?<=[a-z])([A-Z])", replacement=" $1")
## [1] "this Is Camel Case aint It"
The below option should return the same output in both cases.
pat <- "(?<=[a-z])(?=[A-Z])"
str_replace_all(str, pat, " ")
#[1] "this Is Camel Case aint It"
stri_replace_all(str, regex=pat, " ")
#[1] "this Is Camel Case aint It"
According to the help page of ?stri_replace_all, there are examples that suggest $1, $2 are used for replacement
stri_replace_all_regex('123|456|789', '(\\p{N}).(\\p{N})', '$2-$1')
So, it should work if we replace the \\1 with $1
stri_replace_all(str, regex = "(?<=[a-z])([A-Z])", " $1")
#[1] "this Is Camel Case aint It"
Related
I am working with really long strings. How can I remove the first 4 words after a certain string pattern occurs? For example:
string <- "hello I am a user of stackoverflow and I am really happy with all the help the community usually offers when I'm in need of some coding expertise."
#remove the fist 4 words after and including "stackoverflow"
result
"hello I am a user of happy with all the help the community usually offers when I'm in need of some coding expertise."
Solution with base R
A one line solution:
pattern <- "stackoverflow"
string <- "hello I am a user of stackoverflow and I am really happy with all the help the community usually offers when I'm in need of some coding expertise."
gsub(paste0(pattern, "(\\W+\\w+){0,4}\\W*"), "", string)
#> [1] "hello I am a user of happy with all the help the community usually offers when I'm in need of some coding expertise."
How it works
Create the pattern you want with a regex:
"stackoverflow" followed by 4 words.
Definitely, check out ?regex for more info about it.
Words are identified by \\w+ and separators are identified by \\W+ (capital w, it includes spaces and special characters like the apostrophe that you have in the sentence)
(...){0,4} means that the combination of word and separator may repeat up to 4 times.
\\W* needs to identify a possible final separator, so that the remaining two pieces of the sentence won't have two separators dividing them. Try it without, you'll see what I mean.
gsub locates the pattern you want and replace it with "" (thus deliting it).
Handle Exceptions
Note that it works even for particular cases:
# end of a sentence with fewer than 4 words after
string <- "hello I am a user of stackoverflow and I am"
gsub(paste0(pattern, "(\\W+\\w+){0,4}\\W*"), "", string)
#> [1] "hello I am a user of "
# beginning of a sentence
string <- "stackoverflow and I am really happy with all the help the community usually offers when I'm in need of some coding expertise."
gsub(paste0(pattern, "(\\W+\\w+){0,4}\\W*"), "", string)
#> [1] "happy with all the help the community usually offers when I'm in need of some coding expertise."
# pattern == string
string <- "stackoverflow and I am really"
gsub(paste0(pattern, "(\\W+\\w+){0,4}\\W*"), "", string)
#> [1] ""
A tidyverse solution
library(stringr)
# locate start and end position of pattern
tmp <- str_locate(string, paste0(pattern,"(\\W+\\w+){0,4}\\W*"))
# get positions: start_sentence-start_pattern and end_pattern-end_sentence
tmp <- invert_match(tmp)
# get the substrings
tmp <- str_sub(string, tmp[,1], tmp[,2])
# collapse substrings together
str_c(tmp, collapse = "")
#> [1] "hello I am a user of happy with all the help the community usually offers when I'm in need of some coding expertise."
Search for your pattern with additional spaces and words after it. Find the positions of the first last match, split the string and paste it back together. At the end gsub any double (or more) spaces.
string <- "hello I am a user of stackoverflow and I am really happy with all the help the community usually offers when I'm in need of some coding expertise."
pat="stackoverflow"
library(stringr)
tmp=str_locate(
string,
paste0(
pat,
paste0(
rep("\\s?[a-zA-Z]+",4),
collapse=""
)
)
)
gsub("\\s{2,}"," ",
paste0(
substring(string,1,tmp[1]-1),
substring(string,tmp[2]+1)
)
)
[1] "hello I am a user of happy with all the help the community usually offers when I'm in need of some coding expertise."
Quick answer, I am sure you can have better code thant that:
string <- "hello I am a user of stackoverflow and I am really happy with all the help the community usually offers when I'm in need of some coding expertise."
t<-read.table(textConnection(string))
string2<-''
i<-0
j<-0
for(i in 1:length(t)){
if(t[i]=="stackoverflow"){
j=i
}else if(j>0){
if(i-j>4){
string2=paste0(string2, " " , t[i])
}
}else if(j==0){
if(i>1){
string2=paste0(string2, " " , t[i])
}else{
string2=t[i]
}
}
}
print(string2)
This question already has answers here:
R/regex with stringi/ICU: why is a '+' considered a non-[:punct:] character?
(2 answers)
Closed 4 years ago.
I find this really odd :
pattern <- "[[:punct:][:digit:][:space:]]+"
string <- "a . , > 1 b"
gsub(pattern, " ", string)
# [1] "a b"
library(stringr)
str_replace_all(string, pattern, " ")
# [1] "a > b"
str_replace_all(string, "[[:punct:][:digit:][:space:]>]+", " ")
# [1] "a b"
Is this expected ?
Still working on this, but ?"stringi-search-charclass" says:
Beware of using POSIX character classes, e.g. ‘[:punct:]’. ICU
User Guide (see below) states that in general they are not
well-defined, so may end up with something different than you
expect.
In particular, in POSIX-like regex engines, ‘[:punct:]’ stands for
the character class corresponding to the ‘ispunct()’
classification function (check out ‘man 3 ispunct’ on UNIX-like
systems). According to ISO/IEC 9899:1990 (ISO C90), the
‘ispunct()’ function tests for any printing character except for
space or a character for which ‘isalnum()’ is true. However, in a
POSIX setting, the details of what characters belong into which
class depend on the current locale. So the ‘[:punct:]’ class does
not lead to portable code (again, in POSIX-like regex engines).
So a POSIX flavor of ‘[:punct:]’ is more like ‘[\p{P}\p{S}]’ in
‘ICU’. You have been warned.
Copying from the issue posted above,
string <- "a . , > 1 b"
mypunct <- "[[\\p{P}][\\p{S}]]"
stringr::str_remove_all(string, mypunct)
I can appreciate stuff being locale-specific, but it still surprises me that [:punct:] doesn't even work in a C locale ...
I tried to search for the solution, but it appears that there is no clear one for R.
I try to split the string by the pattern of, let's say, space and capital letter and I use stringr package for that.
x <- "Foobar foobar, Foobar foobar"
str_split(x, " [:upper:]")
Normally I would get:
[[1]]
[1] "Foobar foobar," "oobar foobar"
The output I would like to get, however, should include the letter from the delimiter:
[[1]]
[1] "Foobar foobar," "Foobar foobar"
Probably there is no out of box solution in stringr like back-referencing, so I would be happy to get any help.
You may split with 1+ whitespaces that are followed with an uppercase letter:
> str_split(x, "\\s+(?=[[:upper:]])")
[[1]]
[1] "Foobar foobar," "Foobar foobar"
Here,
\\s+ - 1 or more whitespaces
(?=[[:upper:]]) - a positive lookahead (a non-consuming pattern) that only checks for an uppercase letter immediately to the right of the current location in string without adding it to the match value, thus, preserving it in the output.
Note that \s matches various whitespace chars, not just plain regular spaces. Also, it is safer to use [[:upper:]] rather than [:upper:] - if you plan to use the patterns with other regex engines (like PCRE, for example).
We could use a regex lookaround to split at the space between a , and upper case character
str_split(x, "(?<=,) (?=[A-Z])")[[1]]
#[1] "Foobar foobar," "Foobar foobar"
Here are some examples from my data:
a <-c("sp|Q9Y6W5|","sp|Q9HB90|,sp|Q9NQL2|","orf|NCBIAAYI_c_1_1023|",
"orf|NCBIACEN_c_10_906|,orf|NCBIACEO_c_5_1142|",
"orf|NCBIAAYI_c_258|,orf|aot172_c_6_302|,orf|aot180_c_2_405|")
For a: The individual strings can contain even more entries of "sp|" and "orf"
The results have to be like this:
[1] "sp|Q9Y6W5" "sp|Q9HB90,sp|Q9NQL2" "orf|NCBIAAYI_c_1_1023"
"orf|NCBIACEN_c_10_906,orf|NCBIACEO_c_5_1142"
"orf|NCBIAAYI_c_258,orf|aot172_c_6_302,orf|aot180_c_2_405"
So the aim is to remove the last "|" for each "sp|" and "orf|" entry. It seems that "|" is a special challenge because it is a metacharacter in regular expressions. Furthermore, the length and composition of the "orf|" entries varying a lot. The only things they have in common is "orf|" or "sp|" at the beginning and that "|" is on the last position. I tried different things with gsub() but also with the stringr package or regexpr() or [:punct:], but nothing really worked. Maybe it was just the wrong combination.
We can use gsub to match the | that is followed by a , or is at the end ($) of the string and replace with blank ("")
gsub("[|](?=(,|$))", "", a, perl = TRUE)
#[1] "sp|Q9Y6W5"
#[2] "sp|Q9HB90,sp|Q9NQL2"
#[3] "orf|NCBIAAYI_c_1_1023"
#[4] "orf|NCBIACEN_c_10_906,orf|NCBIACEO_c_5_1142"
#[5] "orf|NCBIAAYI_c_258,orf|aot172_c_6_302,orf|aot180_c_2_405"
Or we split by ,', remove the last character withsubstr, andpastethelist` elements together
sapply(strsplit(a, ","), function(x) paste(substr(x, 1, nchar(x)-1), collapse=","))
An easy alternative that might work. You need to escape the "|" using "\\|".
# Input
a <-c("sp|Q9Y6W5|","sp|Q9HB90|,sp|Q9NQL2|","orf|NCBIAAYI_c_1_1023|",
"orf|NCBIACEN_c_10_906|,orf|NCBIACEO_c_5_1142|",
"orf|NCBIAAYI_c_258|,orf|aot172_c_6_302|,orf|aot180_c_2_405|")
# Expected output
b <- c("sp|Q9Y6W5", "sp|Q9HB90,sp|Q9NQL2", "orf|NCBIAAYI_c_1_1023" ,
"orf|NCBIACEN_c_10_906,orf|NCBIACEO_c_5_1142" ,
"orf|NCBIAAYI_c_258,orf|aot172_c_6_302,orf|aot180_c_2_405")
res <- gsub("\\|,", ",", gsub("\\|$", "", a))
all(res == b)
#[1] TRUE
You could construct a single regex call to gsub, but this is simple and easy to understand. The inner gsub looks for | and the end of the string and removes it. The outer gsub looks for ,| and replaces with ,.
You do not have to use a PCRE regex here as all you need can be done with the default TRE regex (if you specify perl=TRUE, the pattern is compiled with a PCRE regex engine and is sometimes slower than TRE default regex engine).
Here is the single simple gsub call:
gsub("\\|(,|$)", "\\1", a)
See the online R demo. No lookarounds are really necessary, as you see.
Pattern details
\\| - a literal | symbol (because if you do not escape it or put into a bracket expression it will denote an alternation operator, see the line below)
(,|$) - a capturing group (referenced to with \1 from the replacement pattern) matching either of the two alternatives:
, - a comma
| - or (the alternation operator)
$ - end of string anchor.
The \1 in the replacement string tells the regex engine to insert the contents stored in the capturing group #1 back into the resulting string (so, the commas are restored that way where necessary).
I am trying to get the text between two words in a sentence.
For example the sentence is -
x <- "This is my first sentence"
Now I want the text between This and first which is is my .
I have tried various functions from R like grep, grepl, pmatch , str_split. However, I could not get exactly what I want .
This is the closest what I have reached with gsub.
gsub(".*This\\s*|first*", "", x)
The output it gives is
[1] "is my sentence"
In reality, what I need is only
[1] "is my"
Any help would be appreciated.
You need .* at the end to match zero or more characters after the 'first'
gsub('^.*This\\s*|\\s*first.*$', '', x)
#[1] "is my"
Another approach using rm_between from the qdapRegex package.
library(qdapRegex)
rm_between(x, 'This', 'first', extract=TRUE)[[1]]
# [1] "is my"
Since this question is used as a reference, I'll add some possible solutions to build a complete overview. Both are based on a look-ahead/look-behind regex pattern.
base R
regmatches( x, gregexpr("(?<=This ).*(?= first)", x, perl = TRUE ) )
stringr
stringr::str_extract_all( x, "(?<=This ).+(?= first)" )