I'm trying to check if a string matches this format:
10_06_13
i.e. todays date, or a similar date with "2digits_2digits_2digits".
What I've done:
regex='([0-9][0-9][_][0-9][0-9][_][0-9][0-9])'
if [[ "$incoming_string" =~ $regex ]]
then
# Do awesome stuff here
fi
This works to a certain extent. But when the incoming string equals 011_100_131, it still passes the regex check. How can I fix my regex to only accept the right format?
=~ succeeds if the string on the left contains a match for the regex on the right. If you want to know if the string matches the regex, you need to "anchor" the regex on both sides, like this:
regex='^[0-9][0-9][_][0-9][0-9][_][0-9][0-9]$'
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:
() from the regex and "" from the [[ ... ]].=~ succeeds if the string matches. 
^ means "excluding" when it is the first thing in a character set ([...]), and it means "anchored" when it is the first thing in a pattern. Otherwise, it just matches ^ (but that's not true in all regex implementations; sometimes it means "match the beginning of a line"). I agree that it's confusing until you get used to it.
_matches the same thing as[_].