如何将表达式的开头和结尾与R中的grep进行匹配

Question

I am trying to match the start and end of expressions with grep command, but I am not able to do that. for example consider the following expressions.

filenames <- c("S2abc.6h", "S2abc.4h", "S2abc.0h","S4abc.6h","S2xyz.6h")

I want to fins all the files starting with S2 and ending with 6h. I can select the files starting with S2 using:

grep("S2", filenames, value = TRUE)

But I am not able to use the wild cards with grep.

> grep("S2*6h", filenames, value = TRUE)

 character(0)

Answer 1

You can use ^ to determine the start and $ to determine the end of a string. The .+ catches everything inbetween.

grep("^S2.+6h$", filenames, value = TRUE)
# [1] "S2abc.6h" "S2xyz.6h"

Answer 2

I think your approach to use grep is fine, you only need to slightly tweak the regular expression you use to match the filenames you want.

> matches <- grep("^S2.*\\.6h$", filenames, ignore.case = T)
> matches
[1] 1 5
> filenames[matches]
[1] "S2abc.6h" "S2xyz.6h"

The regex I used is:

^S2.*\\.6h$

This will match any filename which begins with S2 and ends with .6h