Extract or Replace Matched Substrings

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Description

Extract or replace matched substrings from match data obtained by regexpr, gregexpr or regexec.

Usage

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regmatches(x, m, invert = FALSE)
regmatches(x, m, invert = FALSE) <- value

Arguments

x

a character vector

m

an object with match data

invert

a logical: if TRUE, extract or replace the non-matched substrings.

value

an object with suitable replacement values for the matched or non-matched substrings (see Details).

Details

If invert is FALSE (default), regmatches extracts the matched substrings as specified by the match data. For vector match data (as obtained from regexpr), empty matches are dropped; for list match data, empty matches give empty components (zero-length character vectors).

If invert is TRUE, regmatches extracts the non-matched substrings, i.e., the strings are split according to the matches similar to strsplit (for vector match data, at most a single split is performed).

If invert is NA, regmatches extracts both non-matched and matched substrings, always starting and ending with a non-match (empty if the match occurred at the beginning or the end, respectively).

Note that the match data can be obtained from regular expression matching on a modified version of x with the same numbers of characters.

The replacement function can be used for replacing the matched or non-matched substrings. For vector match data, if invert is FALSE, value should be a character vector with length the number of matched elements in m. Otherwise, it should be a list of character vectors with the same length as m, each as long as the number of replacements needed. Replacement coerces values to character or list and generously recycles values as needed. Missing replacement values are not allowed.

Value

For regmatches, a character vector with the matched substrings if m is a vector and invert is FALSE. Otherwise, a list with the matched or/and non-matched substrings.

For regmatches<-, the updated character vector.

Examples

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x <- c("A and B", "A, B and C", "A, B, C and D", "foobar")
pattern <- "[[:space:]]*(,|and)[[:space:]]"
## Match data from regexpr()
m <- regexpr(pattern, x)
regmatches(x, m)
regmatches(x, m, invert = TRUE)
## Match data from gregexpr()
m <- gregexpr(pattern, x)
regmatches(x, m)
regmatches(x, m, invert = TRUE)

## Consider
x <- "John (fishing, hunting), Paul (hiking, biking)"
## Suppose we want to split at the comma (plus spaces) between the
## persons, but not at the commas in the parenthesized hobby lists.
## One idea is to "blank out" the parenthesized parts to match the
## parts to be used for splitting, and extract the persons as the
## non-matched parts.
## First, match the parenthesized hobby lists.
m <- gregexpr("\\([^)]*\\)", x)
## Write a little utility for creating blank strings with given numbers
## of characters.
blanks <- function(n) strrep(" ", n)
## Create a copy of x with the parenthesized parts blanked out.
s <- x
regmatches(s, m) <- Map(blanks, lapply(regmatches(s, m), nchar))
s
## Compute the positions of the split matches (note that we cannot call
## strsplit() on x with match data from s).
m <- gregexpr(", *", s)
## And finally extract the non-matched parts.
regmatches(x, m, invert = TRUE)

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