How do I match one string from a set of strings, followed by some digits, followed by another string?

Question

I have a set of files with names like this:

01990205abc.dat
20200304abc.dat
20210506abc.csv

My goal is to match the last two strings, but not the first string. I used this pattern: ^(2020)|(2021)[0-9]{4}abc.(csv)|(dat)$ and this code:

files <- list.files(path = "mirror", pattern = "^(2020)|(2021)[0-9]{4}abc.(csv)|(dat)$")
print(files)

matches all three files, instead of just the last two. My expectation was that

• (2020)|(2021) would match one of the years, but not 0199.
• [0-9]{4} would match exactly four digits, then abc, followed by one or the other file extensions.

Answer 1

You could use the pattern

^202[01][0-9]{4}abc\.(?:dat|csv)$

Regex demo

With the doubled escaped dot and capture groups:

^(202[01])[0-9]{4}abc\\.(dat|csv)$

• ^(202[01]) Start of string and capture 2020 or 2021
• [0-9]{4}abc Match 4 digits 0-9 and abc
• \\. Match a dot
• (dat|csv) Capture either dat or csv
• $ End of string

Regex demo