re2r

RE2 is a primarily DFA based regexp engine from Google that is very fast at matching large amounts of text.

Installation

From CRAN:

install.packages("re2r")

From GitHub:

library(devtools)
install_github("qinwf/re2r", build_vignettes = 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

re2_detect(string, pattern) searches the string expression for a pattern and returns boolean result.

test_string = "this is just one test";
re2_detect(test_string, "(o.e)")

## [1] TRUE

Here is an example of email pattern.

show_regex("\\b[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,4}\\b", width = 670, height = 280)

re2_detect("[email protected]", "\\b[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,4}\\b")

## [1] TRUE

re2_match(string, pattern) will return the capture groups in ().

(res = re2_match(test_string, "(o.e)"))

##      .match .1   
## [1,] "one"  "one"

The return result is a character matrix. .1 is the first capture group and it is unnamed group.

Create named capture group with (?P<name>pattern) syntax.

(res = re2_match(test_string, "(?P<testname>this)( is)"))

##      .match    testname .2   
## [1,] "this is" "this"   " is"

is.matrix(res)

## [1] TRUE

is.character(res)

## [1] TRUE

res$testname

## testname 
##   "this"

If there is no capture group, the matched origin strings will be returned.

test_string = c("this is just one test", "the second test");
(res = re2_match(test_string, "is"))

##      .match
## [1,] "is"  
## [2,] NA

re2_match_all() will return the all of patterns in a string instead of just the first one.

res = re2_match_all(c("this is test", 
            "this is test, and this is not test", 
            "they are tests"), 
          pattern = "(?P<testname>this)( is)")
print(res)

## [[1]]
##      .match    testname .2   
## [1,] "this is" "this"   " is"
## 
## [[2]]
##      .match    testname .2   
## [1,] "this is" "this"   " is"
## [2,] "this is" "this"   " is"
## 
## [[3]]
##      .match testname .2

is.list(res)

## [1] TRUE

match all numbers

texts = c("pi is 3.14529..",
          "-15.34 °F",
          "128 days",
          "1.9e10",
          "123,340.00$",
          "only texts")
(number_pattern = re2(".*?(?P<number>-?\\d+(,\\d+)*(\\.\\d+(e\\d+)?)?).*?"))

## re2 pre-compiled regular expression
## 
## pattern: .*?(?P<number>-?\d+(,\d+)*(\.\d+(e\d+)?)?).*?
## number of capturing subpatterns: 4
## capturing names with indices: 
## .match number .2 .3 .4
## expression size: 56

(res = re2_match(texts, number_pattern))

##      .match          number       .2     .3       .4   
## [1,] "pi is 3.14529" "3.14529"    NA     ".14529" NA   
## [2,] "-15.34"        "-15.34"     NA     ".34"    NA   
## [3,] "128"           "128"        NA     NA       NA   
## [4,] "1.9e10"        "1.9e10"     NA     ".9e10"  "e10"
## [5,] "123,340.00"    "123,340.00" ",340" ".00"    NA   
## [6,] NA              NA           NA     NA       NA

res$number

## [1] "3.14529"    "-15.34"     "128"        "1.9e10"     "123,340.00"
## [6] NA

show_regex(number_pattern)

2. Replace a substring

re2_replace(string, pattern, rewrite)

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(new_string, "(o.e)", input_string)

## [1] "my"

mask the middle three digits of a US phone number

texts = c("415-555-1234",
          "650-555-2345",
          "(416)555-3456",
          "202 555 4567",
          "4035555678",
          "1 416 555 9292")

us_phone_pattern = re2("(1?[\\s-]?\\(?\\d{3}\\)?[\\s-]?)(\\d{3})([\\s-]?\\d{4})")

re2_replace(texts, us_phone_pattern, "\\1***\\3")

## [1] "415-***-1234"   "650-***-2345"   "(416)***-3456"  "202 *** 4567"  
## [5] "403***5678"     "1 416 *** 9292"

3. Extract a substring

re2_extract(string, pattern, replacement)

Extract matching patterns from a string.

re2_extract("yabba dabba doo", "(.)")

## [1] "y"

re2_extract("[email protected]", "(.*)@([^.]*)")

## [1] "test@me"

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: 
## .match
## expression size: 10

regexp = re2("test",case_sensitive = FALSE)
re2_match("TEST", regexp)

##      .match
## [1,] "TEST"

re2_replace("TEST", regexp, "ops")

## [1] "ops"

5. Multithread

Use parallel option to enable multithread feature. It will improve performance for large inputs with a multi core CPU.

re2_match(string, pattern, parallel = T)