A match at the start of the string is indicated with character position 1. Each element in the returned vector indicates the character position in each corresponding string element in the input vector at which the (first) regex match was found. regexpr returns an integer vector with the same length as the input vector. The regexpr function takes the same arguments as grepl. Each element in the returned vector indicates whether the regex could find a match in the corresponding string element in the input vector. grepl returns a logical vector with the same length as the input vector. The grepl function takes the same arguments as the grep function, except for the value argument, which is not supported. If you pass value=TRUE, then grep returns a vector with copies of the actual elements in the input vector that could be (partially) matched. If you pass value=FALSE or omit the value parameter then grep returns a new vector with the indexes of the elements in the input vector that could be (partially) matched by the regular expression. The grep function takes your regex as the first argument, and the input vector as the second argument. When using perl=TRUE, as you should, you can add mode modifiers to the start of the regex. R’s functions do not have any parameters to set any other matching modes. You can pass ignore.case=TRUE to make them case insensitive. Starting with R 4.0.0, passing perl=TRUE makes R use the PCRE2 library.Īll the functions use case sensitive matching by default. When this website talks about R, it assumes you’re using the perl=TRUE parameter. This tells R to use the PCRE regular expressions library. The best way to use regular expressions with R is to pass the perl=TRUE parameter. This parameter was deprecated in R 2.10.0 and removed in R 2.11.0. Passing the extended=FALSE parameter allowed you to switch to BRE. Older versions of R used the GNU library to implement both POSIX BRE and ERE.

What this website says about POSIX ERE does not (necessarily) apply to R. It mimics POSIX but deviates from the standard in many subtle and not-so-subtle ways. In R 2.10.0 and later, the default regex engine is a modified version of Ville Laurikari’s TRE engine. The R documentation claims that the default flavor implements POSIX extended regular expressions. The R Project for Statistical Computing provides seven regular expression functions in its base package.

Grep pattern examples