Questions & Help>Questions

Bitstring pattern matching end of string

#bitstring
Fl4m3Ph03n1x

Fl4m3Ph03n1x

Background

We have a simple string of variable size and we want the final 4 characters of the string. Our goal is to achieve this via pattern matching. No special reason, just trying to learn.

Code

This is what we have tried thus far:

<< _, ext :: bytes-4 >> = "my_file_name.exs"
<< _, ".", ext :: bytes-3 >> = "my_file_name.exs"

Trying both lines of code in IEx fails.

Questions

  1. Is it possible to pattern match a bitstring at the end of the string? (we know it works at the start)
  2. Are we doing something wrong? If so, what?
2 comments
#bitstring
1992 2

Showing Posts 2 to 1

hauleth

hauleth

No, not exactly. If you know how many last characters you want to match you can hack it though:

str = "my_file_name.exs"
len = byte_size(str)
<<_::binary-size(len - 4), ?., ext::binary>> = str

But if you want extension, which is can be more than 3 characters (we aren’t using DOS anymore), then Path.extname/1 is the way to go.

5
Post #2
LostKobrakai

LostKobrakai

Only way to do it with pattern matching is reversing, which in itself might not be trivial. For your usecase there’s Path.extname though.

3
Post #1
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