R - regular expression

Question

I would like to list all files in a directory that have "xyz" at the beginning and the first underscore has a 2 after it.

For example, xyzfjd_2_34_1.png, xyz-39_2dog.jpg would work, whereas xyzdog_3_dog.png would not work.

I tried

list.files(dir, pattern="^xyz*_2*");

which is clearly wrong, mostly because I don't even know what I'm doing...

score 3 · Accepted Answer · answered Nov 01 '13 at 00:50

Do file names without an underscore match? I am assuming no.

I think you are interpreting * for any character. Instead, . is any character. * is "0 or more times". So your pattern ^xyz*_2* can match xy_ and xyzzzz_222. It won't match xyz1_2.

Try:

list.files(dir, pattern = "^xyz[^_]*_2")

^ Beginning of the file name
xyz
[^_]* Any combination of characters that is not _ (including the empty string)
_2, an underscore followed by a two.

I don't worry about the rest of the name because the pattern only needs to match part of the file name.

R - regular expression

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