Unix shell function for adding directories to PATH

Adding a directory to the PATH in Unix/Linux is a very common task. However, what if the directory is already in the path? My goal is to write a shell function (preferably portable) that will add a directory to the front or back of the PATH variable, but only if it's not already there.

Here's what I have (syntax is in zsh / bash):

#-------
# DESC: Adds a directory to the PATH if it's not already in the PATH
# ARGS:
#  1 - The directory to add
#  2 - Which end of PATH to add to.  Use "front" to prepend.
#-------
add2path() {
  if ! echo $PATH | egrep "(^|:)$1(:|\$)" > /dev/null ; then
    if [[ $2 = "front" ]]; then
      PATH="$1:$PATH"
    else
      PATH="$PATH:$1"
    fi
    export PATH
  fi
}

This function works. I have tested it on Ubuntu, Solaris, and FreeBSD. And I have tested it in bash, zsh, and ksh. But I want to make sure that it is as portable (first and foremost), readable, and as efficient as possible.

Remarks

• I know that the =~ operator would be more readable, but I had trouble with it not working on certain OSes, particularly Solaris.

• I know I could (and arguably should) use grep -Fq instead of egrep routing to /dev/null, but again this didn't work on certain OSes (Solaris)

• Ironically, the way to fix this on Solaris is to add /usr/xpg4/bin to the PATH. :)

grep is most likely an overkill. After setting IFS=":", the $PATH is conveniently split into words. Then the presence of the directory can be determined in a simple loop

IFS=":"
    for pathdir in $PATH; do
        if [ $pathdir == $1]; then return; fi
    done

    # Now restore IFS and modify path as needed.

StackExchange Code Review Q#88236, answer score: 10