How do regular expressions differ from wildcards used to filter files

Question

While we use * to denote zero or more previous characters in grep, we use *.c to find all C files when we use it with the ls command like ls *.c. Could someone tell how the use of * differs in these two cases?

Answer 1

Shell file name globbing and regular expressions use some of the same characters, and they have similar purposes, but you're right, they aren't compatible. File name globbing is a much less powerful system.

In file name globbing:

• * means "zero or more characters"

• ? means "any single character"

But in regexes, you have to use .* to mean "zero or more characters", and . means "any single character." A ? means something quite different in regexes: zero or one instance of the preceding RE element.

Square brackets ([]) appear to work the same in both systems on the system I'm typing this on, for simple cases at least. This includes things like POSIX character classes (e.g. [:alpha:]). That said, if you need your commands to work on many different system types, I recommend against using anything beyond elementary things like lists of characters (e.g. [abeq]) and maybe character ranges (e.g. [a-c]).

These differences mean the two systems are only directly interchangeable for simple cases. If you need regex matching of file names, you need to do it another way. find -regex is one option. (Notice that there is also find -name, by the way, which uses glob syntax.)

Answer 2

Answering to the question expressed in the original title:

Why do regular expressions differ from that used to filter files?

File name expansion predates regular expressions, already existed with most operating systems (wildcard/joker characters) and is much simpler and intuitive than the latter.

While *.txt is easily understandable by casual users, the analogous .*\.txt is something more targeted to experienced users/programmers, not to mention ^.*\.txt$ ...