Folks;
I have 3 questions & any help with them would be really appreciated:

If i have a list of directories, for example:

/fs/pas/2007/4/6/2634210/admdat/examin
/fs/pas/2007/4/6/2634210/admdat2/stat
/fs/pas/2007/4/6/2634210/admdat3/data
/fs/pas/2007/4/6/2634210/im_2/0b.dcm

Now; my questions are:
1. How to use RE to extract the last file such as "examin and stat,..."?

2. How to use RE to extract the parent directory for the last file. such as "amdat, amdat2, im_2,..."?

3. How to use RE to extract the date in the middle "2007/4/6"?

Are we talking Perl or grep or something else here? There are many different regex flavors and the matching strategy would also depend on what infrastructure is available in the tool.

For Perl, something like m%/(\d{4}/\d{1,2}/\d{1,2})/\d+/([^/]+)/([^/]+)$% would get the substrings you request into $3, $2, and $1, respectively.

Not perl, just normal grep

grep normally returns the entire matching line anyway.

The regular expression should work with grep as such, if you make a few minor substitutions. \d is a Perlism, replace with [0-9]. {4} is an egrep-ism, although POSIX grep has it in some form, too (maybe with backslashes before the braces); or you can simply put the required number of repetitions.

You will be hard pressed to find a situation where you can get exactly only the required parts out of grep, though. [0-9][0-9][0-9][0-9]/[0-9][0-9]/[0-9][0-9] and [^/]*$ will still work, but the penultimate directory I don't think you can get without passing through sed or some such.

I did this below to get the last directory in the tree & all i need is a similar ones:

.*/([^/\n\r]*?)$

If including the last directory component as well is acceptable then you can do [^/]*/[^/]*$ to get the last and the penultimate directory (#1 and half #2) and then if you trim #1 and the last slash, you get the real answer for #2.

*? is certainly not a plain old grep regular expression.

What's the point of this exercise?

I tried your method & it didn't work