sed backreferences and command interpolation - bash

Question

I am having an interesting issue using only sed to substitute short month strings (ex "Oct") with the corresponding number value (ex "10) given a string such as the following:
Oct 14 09:23:35 some other input
To be replaced directly via sed with:
14-10-2013 09:23:25 some other input
None of the following is actually relevant to solving the trivial problem of month string -> number conversion; I'm more interested in understanding some weird behavior I encountered while trying to solve this problem entirely with sed.
Without any attempt of this string substitution (the echo statement is in lieu of the actual input in my script):
...
MMM_DD_HH_mm_SS="([A-Za-z]{3}) ([0-9]{2}) (.+:[0-9]{2})"
echo "Oct 14 09:23:35 some other input" | sed -r "s/$MMM_DD_HH_mm_ss (.+)/\2-\1-\3 \4/"
Then how to transform the backreference \1 into a number. Of course one thinks of using command interpolation with the backreference as an argument:
...
TestFunc()
{
echo "received input $1$1"
}
...
echo "Oct 14 09:23:35 some other input" | sed -r "s/$MMM_DD_HH_mm_ss (.+)/\2-$(TestFunc \\1)-\3 \4/"
Where TestFunc would be a variation of the date command (as proposed by Jotne below) with the echo'd date-time group as an input. Here TestFunc is just an echo because I'm much more interested in the behavior of what the function believes to be the value of $1.
In this case the sed with TestFunc produces the output:
14-received input OctOct-09:23:35 some other input
Which suggests that sed actually is inserting backreference \1 into the command substitution $(...) for handling by TestFunc (which appears to receive \1 as the local variable $1).
However, all attempts to do anything more with the local $1 fail. For example:
TestFunc()
{
echo "processed: $1$1" > tmp.txt # Echo 1
if [ "$1" == "Oct" ]; then
echo "processed: 10"
else
echo "processed: $1$1" # Echo 2
fi
}
Returns:
14-processed: OctOct-09:23:35 some other input
$1 has been substituted into Echo 2, yet tmp.txt contains the value processed: \1\1; as if the backreference is not being inserted into the command substitution. Even weirder, the if condition fails with $1 != "Oct", yet it falls through to an echo statement which indicates $1 = "Oct".
My question is why is the backreference insertion working in the case of Echo 2 but not Echo 1? I suspect that the backreference insertion isn't working at all (given the failure of the if statement in TestFunc) but rather something subtle is going on that makes the substitution appear to work correctly in the case of Echo 2; what is that subtlety?
Solution
On further reflection I believe I understand what is going on:
\\1 is passed to the command substitution subroutine / child function as the literal \1. This is why equality test within the child function is failing.
however the echo function is correctly handling the string \\1 as $1. So echo "aa$1aa" returns the result of the command substitution to sed as aa\1aa. Other functions such as rev also "see" $1 as \1.
sed then interpolates \1 in aa\1aa as Oct or whatever the backreference is, to return aaOctaa to the user.
Since command substitution within regexes clearly works, it would be really cool if sed replaced the value of \\1 (or \1, whatever) with the backreference before executing the command substitution $(...); this would significantly increase sed's power...

Answer 1

This might work for you (GNU sed):
s/$/\nJan01...Oct10Nov11Dec12/;s/(...) (..) (..:..:.. .*)\n.*\1(..).*/\2-\4-2013 \3/;s/\n.*//' file
Add a lookup to the end of the line and use the back reference to match on it making sure to remove the lookup table in all cases.
Here's an example of passing a backreference to a function:
f(){ echo "x$1y$1z"; }
echo a b c | sed -r 's/(.) (.) (.)/'"$(f \\2)"'/'
returns:
xbybz
HTH

Answer 2

Use the correct tool:
date -d "Oct 14 09:23:35" +"%d-%m-%Y %H:%M:%S"
14-10-2013 09:23:35
Date does read your input and convert it to any format you like