Matching A File Name Using Grep - regex

Question

The overarching problem:
So I have a file name that comes in the form of
JohnSmith14_120325_A10_6.raw
and I want to match it using regex. I have a couple of issues in building a working example but unfortunately my issues won't be solved unless I get the basics.
So I have just recently learned about piping and one of the cool things I learned was that I can do the following.
X=ll_paprika.sc (don't ask)
VAR=`echo $X | cut -p -f 1`
echo $VAR
which gives me paprika.sc
Now when I try to execute the pipe idea in grep, nothing happens.
x=ll_paprika.sc
VAR=`echo $X | grep *.sc`
echo $VAR
Can anyone explain what I am doing wrong?
Second question:
How does one match a single underscore using regex?
Here's what I am ultimately trying to do;
VAR=`echo $X | grep -e "^[a-bA-Z][a-bA-Z0-9]*(_){1}[0-9]*(_){1}[a-bA-Z0-9]*(_){1}[0-9](\.){1}(raw)"
So the basic idea of my pattern here is that the file name must start with a letter
and then it can have any number of letters and numbers following it and it must have an _ delimit a series of numbers and another _ to delimit the next set of numbers and characters and another _ to delimit the next set of numbers and then it must have a single period following by raw. This looks grossly wrong and ugly (because I am not sure about the syntax). So how does one match a file extension? Can someone put up a simple example for something ll_parpika.sc so that I can figure out how to do my own regex?
Thanks.

Answer 1

x=ll_paprika.sc
VAR=`echo $X | grep *.sc`
echo $VAR
The reason this isn't doing what you want is that the grep matches a line and returns it. *.sc does in fact match 11_paprika.sc, so it returns that whole line and sticks it in $VAR.
If you want to just get a part of it, the cut line probably better. There is a grep -o option that returns only the matching portion, but for this you'd basically have to put in the thing you were looking for, at which point why bother?
the file name must start with a letter
`grep -e "^[a-zA-Z]
and then it can have any number
of letters and numbers following it
[a-zA-Z0-9]*
and it must have an _ delimit a
series of numbers and another _ to delimit the next set of numbers and
characters and another _ to delimit the next set of numbers
(_[0-9]+){3}
and then it must have a single period following by raw.
.raw"

Answer 2

For the first, use:
VAR=`echo $X | egrep '\.sc$'`
For the second, you can try this alternative instead:
VAR=`echo $X | egrep '^[[:alpha:]][[:alnum:]]*_[[:digit:]]+_[[:alnum:]]+_[[:digit:]]+\.raw'`
Note that your character classes from your expression differ from the description that follows in that they seem to only be permissive of a-b for lower case characters in some places. This example is permissive of all alphanumeric characters in those places.