lookahead: Lookaround

In rebus.base: Core Functionality for the 'rebus' Package

Description

Zero length matching. That is, the characters are matched when detecting, but not matching or extrcting.

Usage

lookahead(x)

negative_lookahead(x)

lookbehind(x)

negative_lookbehind(x)

Arguments

x	A character vector.

Value

A character vector representing part or all of a regular expression.

Note

Lookbehind is not supported by R's PRCE engine. Use R's Perl engine or stringi/stringr's ICU engine.

References

http://www.regular-expressions.info/lookaround.html and http://www.rexegg.com/regex-lookarounds.html

Examples

x <- "foo"
lookahead(x)
negative_lookahead(x)
lookbehind(x)
negative_lookbehind(x)

# Usage
x <- c("mozambique", "qatar", "iraq")
# q followed by a character that isn't u
(rx_neg_class <- "q" %R% negated_char_class("u"))
# q not followed by a u
(rx_neg_lookahead <- "q" %R% negative_lookahead("u"))
stringi::stri_detect_regex(x, rx_neg_class)
stringi::stri_detect_regex(x, rx_neg_lookahead)
stringi::stri_extract_first_regex(x, rx_neg_class)
stringi::stri_extract_first_regex(x, rx_neg_lookahead)

# PRCE engine doesn't support lookbehind
x2 <- c("queen", "vacuum")
(rx_lookbehind <- lookbehind("q")) %R% "u"
stringi::stri_detect_regex(x2, rx_lookbehind)
try(grepl(rx_lookbehind, x2))
grepl(rx_lookbehind, x2, perl = TRUE)

Example output

<regex> (?=foo)
<regex> (?!foo)
<regex> (?<=foo)
<regex> (?<!foo)
<regex> q[^u]
<regex> q(?!u)
[1] FALSE  TRUE FALSE
[1] FALSE  TRUE  TRUE
[1] NA   "qa" NA  
[1] NA  "q" "q"
<regex> (?<=q)u
[1]  TRUE FALSE
Error in grepl(rx_lookbehind, x2) : 
  invalid regular expression '(?<=q)', reason 'Invalid regexp'
[1]  TRUE FALSE

rebus.base documentation built on May 2, 2019, 5:14 a.m.