R: Lookarounds

lookarounds {rex} | R Documentation

## Lookarounds

### Description

Lookarounds

### Usage

    x %if_next_is% y

    x %if_next_isnt% y

    x %if_prev_is% y

    x %if_prev_isnt% y

### Arguments

`x` | A regex pattern.
`y` | A regex pattern.

### Details

These functions provide an interface to perl lookarounds.

Special binary functions are used to infer an ordering, since often you might wish to match a word / set of characters conditional on the start and end of that word.

- `%if_next_is%`: `TRUE` if x follows y

- `%if_next_isnt%`: `TRUE` if x does not follow y

- `%if_prev_is%`: `TRUE` if y comes before x

- `%if_prev_isnt%`: `TRUE` if y does not come before x

### See Also

Perl 5 Documentation http://perldoc.perl.org/perlre.html#Extended-Patterns

Other rex: `%or%`, `capture`, `character_class`, `counts`, `group`, `not`, `rex`, `shortcuts`, `wildcards`

### Examples

    stopifnot(grepl(rex("crab" %if_next_is% "apple"), "crabapple", perl = TRUE))
    stopifnot(grepl(rex("crab" %if_prev_is% "apple"), "applecrab", perl = TRUE))
    stopifnot(grepl(rex(range("a", "e") %if_next_isnt% range("f", "g")),
      "ah", perl = TRUE))
    stopifnot(grepl(rex(range("a", "e") %if_next_is% range("f", "i")),
      "ah", perl = TRUE))

[Package rex version 1.1.2 Index]