Regex match exact number of letters

Question

Let\'s say I want to find all words in which letter \"e\" appears exactly two times. When I define this pattern:

pattern1 <- \"e.*e\" 
grep(pattern1, stri         


        

Answer 1

We can use a pattern to match zero or more characters that are not 'e' ([^e]*) from the start (^) of the string, followed by character 'e', then another set of characters that are not 'e' followed by 'e', and zero or more characters not an 'e' until the end ($) of the string

res <- grep("^[^e]*e[^e]*e[^e]*$", stringr::words, value = TRUE)
stringr::str_count(res, "e")
#[1] 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
#[58] 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
#[115] 2 2 2 2 2 2 2

Answer 2

If you're okay not using grep

stringr::str_count(words, "e") == 2

If you want more efficiency,

stringi::stri_count_fixed(words, "e") == 2

Both of these return logical vectors, you can get the words with words[..code from above..]

Answer 3

^[^e]*e[^e]e[^e]$

^ asserts :: start of the string

[^e]* :: Match a zero or more character not present in the list

*(asterisk) — Matches between zero and unlimited times, as many times as possible

e :: matches the character e literally (case sensitive)

repeat [^e]* to match all other characters if between 2 e's

$ asserts position at the end of the string, or before the line terminator right at the end of the string (if any)

so, [^e]* matches all characters except e, zero or multiple times. so that if string contain only e then also condition satisfy as it consider zero occurrence of all other characters.

Answer 4

You may use:

^(?:[^e]*e){2}[^e]*$

See the regex demo. The (?:...) is a non-capturing group that allows quantifying a sequence of subpatterns and is thus easily adjustable to match 3, 4 or more specific sequences in a string.

Details

• ^- start of string
• (?:[^e]*e){2} - 2 occurrences of
  • [^e]* - any 0+ chars other than e
  • e - an e
• [^e]* - any 0+ chars other than e
• $ - end of string

See the R demo below:

x <- c("feel", "agre", "degree")
rx <- "^(?:[^e]*e){2}[^e]*$"
grep(rx, x, value = TRUE)
## => [1] "feel"

Note that instead of value = T it is safer to use value = TRUE as T might be redefined in the code above.