re2r
RE2 is a primarily DFA based regexp engine from Google that is very fast at matching large amounts of text.
Installation
To install from GitHub:
library(devtools)
install_github("rstudio/rmarkdown")
install_github("yihui/knitr")
install_github("qinwf/re2r", build_vignettes = T, force = T)To learn how to use, you can check out the vignettes.
Related Work
Google Summer of Code - re2 regular expressions.
Brief Intro
1. Search a string for a pattern
## Sys.setlocale(locale = "English") ## for Windows users with non-UTF8 locale
## re2_match(pattern, string)
test_string = "this is just one test";
re2_match("(o.e)", test_string)## [1] TRUESearches the string expression for the occurence(s) of a substring that matches 'pattern' and returns boolean result.
With value = TRUE option, function will return the capture groups with ().
(res = re2_match("(o.e)", test_string, value = TRUE))
## ?1
##[1,] "one"
str(res)
## chr [1, 1] "one"
## - attr(*, "dimnames")=List of 2
## ..$ : NULL
## ..$ : chr "?1"The return result is a character matrix. ?1 is the first capture group and it is unnamed group.
We can create named capture group with (?P<name>pattern) syntax.
(res = re2_match("(?P<testname>this)( is)", test_string, value = TRUE))
## testname ?2
##[1,] "this" " is"
str(res)
##[1] "matrix"With all = TRUE option, function will return the all of patterns in a string instead of just the first one.
(res = re2_match("(is)", test_string, value = TRUE, all = TRUE))## !n ?1
##[1,] "1" "is"
##[2,] "1" "is"!n is the index of the input string. We can provide a character vector to a pattern.
test_string = c("this is just one test", "the second test");
(res = re2_match("(is)", test_string, value = TRUE, all = TRUE))## !n ?1
##[1,] "1" "is"
##[2,] "1" "is"
##[3,] "2" NA If there is no capture group, and value = TRUE, the matched origin strings will be returned.
test_string = c("this is just one test", "the second test");
(res = re2_match("is", test_string, value = TRUE))## ?nocapture
##[1,] "this is just one test"
##[2,] NA 2. Replace a substring
## re2_replace(pattern, rewrite, input)Searches the string "input string" for the occurence(s) of a substring that matches 'pattern' and replaces the found substrings with "rewrite text".
input_string = "this is just one test";
new_string = "my"
re2_replace("(o.e)", new_string, input_string)## [1] "this is just my test"3. Extract a substring
## re2_extract(pattern, input, rewrite = optional)Searches the string "input string" for the occurence(s) of a substring that matches 'pattern' and return the found substrings with "rewrite text".
re2_extract("(.)","yabba dabba doo")## [1] "y"re2_extract("(.*)@([^.]*)","test@me.com","\\2!\\1")## [1] "me!test"\\1 and \\2 are the first and second capture groups.
4. Regular Expression Object for better performance
We can create a regular expression object (RE2 object) from a string. It will reduce the time to parse the syntax of the same pattern.
And this will also give us more option for the pattern. run help(re2) to get more detials.
regexp = re2("test",case_sensitive = FALSE)
print(regexp)
## re2 pre-compiled regular expression
##
## pattern: test
## number of capturing subpatterns: 0
## capturing names with indices:
## named integer(0)
## expression size: 11
regexp %<~% "(?P<first>1*)"
regexp
## re2 pre-compiled regular expression
##
## pattern: (?P<first>1*)
## number of capturing subpatterns: 1
## capturing names with indices:
## first
## 1
## expression size: 8%<~% is a operator to create a new RE2 object.
regexp = re2("test",case_sensitive = FALSE)
re2_match(regexp, "TEST")
## [1] TRUE
re2_replace(regexp, "ops", "TEST")
## [1] "ops"If you come from a Perl world, you may be insterested in %=~% %!~%.
"TEST" %=~% regexp
## [1] TRUE
"TEST" %!~% regexp
## [1] FALSE
