The use of dollar sign '$' after file type when listing files

Question

I have seen a lot of people using the dollar sign $ after .csv when listing a bunch of files, e.g.

filenames <- list.files(path='fake/path/name',pattern='*.csv$')

However, when I was working on a project, it worked for me without the dollar sign. My code is

filenames <- list.files(path='fake/path/name',pattern='*.csv')

I haven't found a clear explanation of what this dollar sign does exactly in this situation. All of my files were listed in both usages. Does anyone know what difference this dollar sign makes?

user_190277 · Answer

The dollar sign is an end-of-line marker in a regular expression. pattern='*.csv$' matches only file names that end with ".csv", so that e.g. if you had backup files called foo.csv.bak or foo.csv~, these wouldn't get included.

user_3732271 · Answer

The $ is a metacharacter in regex to specify the end ($) of the string. Usually the csv will be at the end of the file name. Thus, it works. But, there are certain cases, where we may find a random csv character and gives erroneous results.

In addition, the OP was using .csv. Here, again, this is problematic as . is a metacharacter that matches any other character. A more correct version will be \.csv$ i.e. escape the . to evaluate it literally

user_7199966 · Answer

This is more of an regular expression (regex) question, the $ at the end means that you are ensuring that the csv stays at the end, it is called anchoring, you can read a great cheatsheet here: https://evoldyn.gitlab.io/evomics-2018/ref-sheets/R_strings.pdf

And to learn more about regex you can just use this excelent resource https://regexr.com/

The use of dollar sign '$' after file type when listing files

3 Answers