asterisk(*) and dot-asterisk(.*) in unix regular expression
regex, unix
Solution
You're talking about two separate kinds of patterns in UNIX, the glob (wildcard) and the regular expression.
Globs are typically used when matching filenames. Your shell does wildcard expansion automatically on command arguments, which is how you can write
ls *foo*
to list all files containing the string `foo`. Globs are very simple (ignoring `extglob`): the syntax typically only supports `*` (any characters) and `?` (any one character). Because of the shell's automatic expansion, these can be used anywhere and must be explicitly suppressed to avoid unwanted matching (e.g. writing `grep '.*foo.*'` to avoid expanding the `*` in the pattern).
On the other hand, we have regular expressions, which are used to pattern-match text. Typically, these are used with specific utilities like `grep`, `sed` and `perl` which understand these patterns, though some shells have contexts which take regex (e.g. Bash's `${x/pat/sub}`). Regular expression syntax is much more powerful, which makes it ideal for looking through reams of text for specific patterns.
Problem
I am always finding myself confused with the usage of these two wildcards in UNIX. I understand asterisk can be used only as a quantifier in a regexp, but UNIX allows `*` in some cases like the `ls` command whereas in `grep` it should be used only as a quantifier. Is there some rule out there which will guide me which one of these to use?