Extract variable value from file

gistfile1.sh

This should extract the value of a variable from a text file.
It works fine with bash/sh and makefile variables.
Had to use 2 sed()s to deal with name='value' cases.
Fixes are welcome.

  $ VAR_NAME=PREFIX
  $ sed --version
  GNU sed version 4.1.5
  $ cat filename.conf | \
    sed -ne \
    "s%^[[:space:]]*\(export[[:space:]]\+\)\?$VAR_NAME[[:space:]]*[:?+]\?=[[:space:]]*['\"]\([^'\"]\+\)['\"].*%\2%p" \
    || \
    sed -ne \
    "s%^[[:space:]]*\(export[[:space:]]\+\)\?$VAR_NAME[[:space:]]*[:?+]\?=[[:space:]]*\([^'\"#[:space:]]\+\).*%\2%p"


=== Example file ===
 PREFIX=a-1
 PREFIX = a-2
 PREFIX =a-3
 PREFIX= a-4
 PREFIX='a 5'
 PREFIX = 'a seis'
PREFIX ='a sete'
PREFIX= 'a oito'
PREFIX="a nove"
PREFIX = "a dez"
PREFIX ="a onze"
PREFIX= "a doze"

PREFIX=b-1       # comentario
PREFIX = b-2       # comentario
PREFIX =b-3       # comentario
PREFIX= b-4       # comentario
PREFIX='b 5'       # comentario
PREFIX = 'b seis'       # comentario
 PREFIX ='b sete'       # comentario
 PREFIX= 'b oito'       # comentario
 PREFIX="b nove"       # comentario
 PREFIX = "b dez"       # comentario
 PREFIX ="b onze"       # comentario
 PREFIX= "b doze"       # comentario

export PREFIX=c-1
export PREFIX = c-2
export PREFIX =c-3
export PREFIX= c-4
export PREFIX='c 5'
export PREFIX = 'c seis'
export  PREFIX ='c sete'
export  PREFIX= 'c oito'
export  PREFIX="c nove"
export  PREFIX = "c dez"
export  PREFIX ="c onze"
export  PREFIX= "c doze"

 export PREFIX=d-1        # comentario
 export PREFIX = d-2        # comentario
 export PREFIX =d-3        # comentario
 export PREFIX= d-4        # comentario
 export PREFIX='d 5'        # comentario
 export PREFIX = 'd seis'        # comentario
export PREFIX ='d sete'        # comentario
export PREFIX= 'd oito'        # comentario
export PREFIX="d nove"        # comentario
export PREFIX = "d dez"        # comentario
export PREFIX ="d onze"        # comentario
export PREFIX= "d doze"        # comentario

export PREFIX :='--d sete'        # comentario
export PREFIX?= '--d oito'        # comentario
export PREFIX+="--d nove"        # comentario
export PREFIX := "--d dez"        # comentario
export PREFIX ?="--d onze"        # comentario
export PREFIX:= "--d doze"        # comentario

=== End example file ===