Q:

Match letters in R regex

Suppose I run the following
txt <- "client:A, field:foo, category:bar"
grep("field:[A-z]+", txt, value = TRUE, perl = TRUE)

Based on regexr.com I expected I would get field:foo, but instead I get the entire string. Why is this?

A:

You seem to want to extract the value. Use regmatches:
txt <- "client:A, field:foo, category:bar"
regmatches(txt, regexpr("field:[[:alpha:]]+", txt))
# => [1] "field:foo"

See the R demo.
To match multiple occurrences, replace regexpr with gregexpr.
Or use stringr str_extract_all:
library(stringr)
str_extract_all(text, "field:[a-zA-Z]+")

Another point is that [A-z] matches more than ASCII letters. Use [[:alpha:]] in a TRE (regexpr / gregexpr with no perl=TRUE)/ICU (stringr) regex to match any letter.

