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

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.