Remove any digit only in first N characters

Question

I'm looking for a regular expression to catch all digits in the first 7 characters in a string.

This string has 12 characters:

A12B345CD678

I would like to remove A and B only since they are within the first 7 chars (A12B345) and get

12345CD678

So, the CD678 should not be touched. My current solution in R:

paste(paste(str_extract_all(substr("A12B345CD678",1,7), "[0-9]+")[[1]],collapse=""),substr("A12B345CD678",8,nchar("A12B345CD678")),sep="‌")

It seems too complicated. I split the string at 7 as described, match any digits in the first 7 characters and bind it with the rest of the string.

Looking for a general answer, my current solution is to split the first 7 characters and just match all digits in this sub string.

Any help appreciated.

Your question doesn't make any sense to me. `12345CD678` is more than 7 characters, even more than 7 digits - and what do you mean by `the first E-F`? — Tim Pietzcker, Feb 15 '16 at 13:23
Your example is confusing. What is "first E-F 7 characters"? Why do you also match CD even though they are not digits? — neuhaus, Feb 15 '16 at 13:26
To be more clear, A12B345CD678 has 12 characters. I want to match all digits in the first 7 characters, so CD678 would not be touched.. My current solution in R: paste(paste(str_extract_all(substr("A12B345CD678",1,7), "[0-9]+")[[1]],collapse=""),substr("A12B345CD678",8,nchar("A12B345CD678")),sep="") which seems to be to complicated. I split the string at 7 as descriped, match any digets in the first 7 characters and bind it with the rest of the string. — Sebastian, Feb 15 '16 at 13:40
Seems like you need variable length lookbehind [like this demo](http://regexstorm.net/tester?p=(%3f%3c%3d%5e.%7b0%2c6%7d)%5cD&i=A12B345CD678&o=m): `(?<=^.{0,6})\D` which is not available in R regex flavor. [This could work](https://regex101.com/r/uS0oF0/1) with `perl=TRUE` but looks even worse to maintain. — bobble bubble, Feb 15 '16 at 13:59
I see this is more complex then I thought, thank you for your efforts bobble bubble — Sebastian, Feb 15 '16 at 14:02

score 14 · Accepted Answer · edited May 23 '17 at 11:45

14

You can use the known SKIP-FAIL regex trick to match all the rest of the string beginning with the 8th character, and only match non-digit characters within the first 7 with a lookbehind:

s <- "A12B345CD678"
gsub("(?<=.{7}).*$(*SKIP)(*F)|\\D", "", s, perl=T)
## => [1] "12345CD678"

See IDEONE demo

The perl=T is required for this regex to work. The regex breakdown:

(?<=.{7}).*$(*SKIP)(*F) - matches any character but a newline (add (?s) at the beginning if you have newline symbols in the input), as many as possible (.*) up to the end ($, also \\z might be required to remove final newlines), but only if preceded with 7 characters (this is set by the lookbehind (?<=.{7})). The (*SKIP)(*F) verbs make the engine omit the whole matched text and advance the regex index to the position at the end of that text.
| - or...
\\D - a non-digit character.

See the regex demo.

edited May 23 '17 at 11:45

Community

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answered Feb 15 '16 at 14:07

Wiktor Stribiżew

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Great idea thought out of the box Stribiżew! – bobble bubble Feb 15 '16 at 14:10
Works perfect, thank you especially for the explanation. – Sebastian Feb 15 '16 at 14:48
Just in case there are newlines, here is how the regex would look like: [`"(?s)(?<=.{7}).*\\z(*SKIP)(*F)|\\D"`](https://ideone.com/CXQkZE). – Wiktor Stribiżew Feb 15 '16 at 14:53

score 5 · Answer 2 · answered Nov 20 '17 at 21:57

5

The regex solution is cool, but I'd use something easier to read for maintainability. E.g.

library(stringr)

str_sub(s, 1, 7) = gsub('[A-Z]', '', str_sub(s, 1, 7))

answered Nov 20 '17 at 21:57

eddi

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should [A-Z] be [A-Za-z]? – caot Nov 24 '17 at 16:20

score 2 · Answer 3 · answered Nov 25 '17 at 09:56

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You can also use a simple negative lookbehind:

s <- "A12B345CD678"
gsub("(?<!.{7})\\D", "", s, perl=T)

answered Nov 25 '17 at 09:56

Casimir et Hippolyte

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Remove any digit only in first N characters

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