如果下划线在位置 3 之后，则 R 正则表达式删除下划线之后的所有内容

Question

I have been searching for a solution for two days.

Here is a sample of what my data looks like and what I would like to achieve:

dat <- c("f__dfty","fd_fgtekg","f_glgkt_s2","f_glgkt_s3","fthssfy_s2","fthssfy_s3","h__gkdnt_s2","sedfgrtsd")
dat <- c("f__dfty","fd_fgtekg","f_glgkt","f_glgkt","fthssfy","fthssfy","h__gkdnt","sedfgrtsd")

I need to remove everything after an "_", but not if the underscore is in position 2 and or 3 of the string. Not every string will have an underscore.

Thanks!

Answer 1

Brief

Not sure about length of strings, so I'll assume any length can be used.

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Code

See this code in use here

Regex

^((?:.{3})?[^_\s]+).*$

Note: You can actually use ^((?:.{3})?[^_]+).*$ instead, but since my example on regex101 uses multiline input to simplify things, I posted the code I used there.

Substitution

$1

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Results

Input

f__dfty
fd_fgtekg
f_glgkt_s2
f_glgkt_s3
fthssfy_s2
fthssfy_s3
h__gkdnt_s2
sedfgrtsd
aaaaaaa_aaaa

Output

f__dfty
fd_fgtekg
f_glgkt
f_glgkt
fthssfy
fthssfy
h__gkdnt
sedfgrtsd
aaaaaaa

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Explanation

• Assert position at beginning of line ^
• Capture the following
  • Optional match of any character 3 times (?:.{3})?
  • Match between 1 and unlimited of any character not present in the set _\\s ( \\s to prevent newline matches in example on regex101; this can be removed from your code if looping through an array/list/etc.) [^_\\s]+
• Match any character any number of times .*
• Assert position at the end of the line $
• Replace with first capture group $1