# Introduction to re2

# re2: R interface to Google RE2

## Overview

re2 package provides pattern matching, extraction, replacement and other string processing operations using Google’s RE2 (C++) regular-expression library. The interface is consistent, and similar to stringr.

Why re2?

Regular expression matching can be done in two ways: using recursive backtracking or using finite automata-based techniques.

Perl, PCRE, Python, Ruby, Java, and many other languages rely on recursive backtracking for their regular expression implementations. The problem with this approach is that performance can degrade very quickly. Time complexity can be exponential. In contrast, re2 uses finite automata-based techniques for regular expression matching, guaranteeing linear time execution and a fixed stack footprint. See links to Russ Cox’s excellent articles below.

## Installation

    # Install the released version from CRAN:
    install.packages("re2")

    # Install the development version from GitHub:
    # install.packages("devtools")
    devtools::install_github("girishji/re2")

## Usage

re2 provides three types of regular-expression functions:

- Find the presence of a pattern in string
- Extract substrings that match a pattern
- Replace matched groups

All functions take a vector of strings as argument. Regular-expression patterns can be compiled, and reused for performance.

Here are the primary verbs of re2:

- `re2_detect(x, pattern)` finds if a pattern is present in string

    re2_detect(c("barbazbla", "foobar", "foxy brown"), "(foo)|(bar)baz")
    #> [1]  TRUE  TRUE FALSE

- `re2_count(x, pattern)` counts the number of matches in string

    re2_count(c("yellowgreen", "steelblue", "maroon"), "e")
    #> [1] 3 3 0

- `re2_subset(x, pattern)` selects strings that match

    re2_subset(c("yellowgreen", "steelblue", "goldenrod"), "ee")
    #> [1] "yellowgreen" "steelblue"

- `re2_match(x, pattern, simplify = FALSE)` extracts first matched substring

    re2_match("ruby:1234 68 red:92 blue:", "(\\w+):(\\d+)")
    #>      .0          .1     .2
    #> [1,] "ruby:1234" "ruby" "1234"

    # Groups can be named:

    re2_match(c("barbazbla", "foobar"), "(foo)|(?P<TestGroup>bar)baz")
    #>      .0       .1    TestGroup
    #> [1,] "barbaz" NA    "bar"
    #> [2,] "foo"    "foo" NA

    # Use pre-compiled regular expression:

    re <- re2_regexp("(foo)|(bar)baz", case_sensitive = FALSE)
    re2_match(c("BaRbazbla", "Foobar"), re)
    #>      .0       .1    .2
    #> [1,] "BaRbaz" NA    "BaR"
    #> [2,] "Foo"    "Foo" NA

- `re2_match_all(x, pattern)` extracts all matched substrings

    re2_match_all("ruby:1234 68 red:92 blue:", "(\\w+):(\\d+)")
    #> [[1]]
    #>      .0          .1     .2
    #> [1,] "ruby:1234" "ruby" "1234"
    #> [2,] "red:92"    "red"  "92"

- `re2_replace(x, pattern, rewrite)` replaces first matched pattern in string

    re2_replace("yabba dabba doo", "b+", "d")
    #> [1] "yada dabba doo"

    # Use groups in rewrite:

    re2_replace("bunny@wunnies.pl", "(.*)@([^.]*)", "\\2!\\1")
    #> [1] "wunnies!bunny.pl"

- `re2_replace_all(x, pattern, rewrite)` replaces all matched patterns in string

    re2_replace_all("yabba dabba doo", "b+", "d")
    #> [1] "yada dada doo"

- `re2_extract_replace(x, pattern, rewrite)` extracts and substitutes (ignores non-matching portions of x)

    re2_extract_replace("bunny@wunnies.pl", "(.*)@([^.]*)", "\\2!\\1")
    #> [1] "wunnies!bunny"

- `re2_split(x, pattern, simplify = FALSE, n = Inf)` splits string based on pattern

    re2_split("How vexingly quick daft zebras jump!", " quick | zebras")
    #> [[1]]
    #> [1] "How vexingly" "daft"         " jump!"

- `re2_locate(x, pattern)` seeks the start and end of pattern in string

    re2_locate(c("yellowgreen", "steelblue"), "l(b)?l")
    #>      begin end
    #> [1,]     3   4
    #> [2,]     5   7

- `re2_locate_all(x, pattern)` locates start and end of all occurrences of pattern in string

    re2_locate_all(c("yellowgreen", "steelblue"), "l")
    #> [[1]]
    #>      begin end
    #> [1,]     3   3
    #> [2,]     4   4
    #>
    #> [[2]]
    #>      begin end
    #> [1,]     5   5
    #> [2,]     7   7

In all the above functions, regular-expression pattern is vectorized.

Regular-expression pattern can be compiled using `re2_regexp(pattern, ...)`. Here are some of the options:

- `case_sensitive`: Match is case-sensitive
- `encoding`: UTF8 or Latin1
- `literal`: Interpret pattern as literal, not regexp
- `longest_match`: Search for longest match, not first match
- `posix_syntax`: Restrict regexps to POSIX egrep syntax

`help(re2_regexp)` lists available options.

`re2_get_options(regexp_ptr)` returns a list of options stored in the compiled regular-expression object.

## Regexp Syntax

re2 supports pearl style regular expressions (with extensions like \d, \w, \s, …) and provides most of the functionality of PCRE – eschewing only backreferences and look-around assertions.

See RE2 Syntax for the syntax supported by RE2, and a comparison with PCRE and PERL regexps.

For those not familiar with Perl’s regular expressions, here are some examples of the most commonly used extensions:

`"hello (\\w+) world"` | `\w` matches a “word” character
`"version (\\d+)"` | `\d` matches a digit
`"hello\\s+world"` | `\s` matches any whitespace character
`"\\b(\\w+)\\b"` | `\b` matches non-empty string at word boundary
`"(?i)hello"` | `(?i)` turns on case-insensitive matching
`"/\\*(.*?)\\*/"` | `.*?` matches . minimum no. of times possible

The double backslashes are needed when writing R string literals. However, they should not be used when writing raw string literals:

`r"(hello (\w+) world)"` | `\w` matches a “word” character
`r"(version (\d+))"` | `\d` matches a digit
`r"(hello\s+world)"` | `\s` matches any whitespace character
`r"(\b(\w+)\b)"` | `\b` matches non-empty string at word boundary
`r"((?i)hello)"` | `(?i)` turns on case-insensitive matching
`r"(/\*(.*?)\*/)"` | `.*?` matches `.` minimum no. of times possible

## References

- Regular Expression Matching Can Be Simple And Fast
- Regular Expression Matching: the Virtual Machine Approach
- Regular Expression Matching in the Wild
- RE2 Syntax