Regular Expression in Bash Script

=~ succeeds if the string on the left contains a match for the regex on the right. If you want to know if the string matches the regex, you need to “anchor” the regex on both sides, like this:

regex='^[0-9][0-9][_][0-9][0-9][_][0-9][0-9]$'
if [[ $incoming_string =~ $regex ]]
then
  # Do awesome stuff here
fi

The ^ only succeeds at the beginning of the string, and the $ only succeeds at the end.

Notes:

1. I removed the unnecessary () from the regex and "" from the [[ ... ]].
2. The bash manual is poorly worded, since it says that =~ succeeds if the string matches.