Regular expression for extracting date - regex

Question

I a regular expression to match a date on the form 01/Jan/2000:23:59:59. I managed to match the pattern using Notepad++'s regex interpreter, using the following regex:
[1-3][0-9]/[A-Z][a-z]{2}/(19|20)[0-9]{2}:[0-9]{2}:[0-5][0-9]:[0-5][0-9]
Unfortunately, I need to do this with bash. AWK is not an option right now, I'm afraid. So, I tried to convert the above regex into something that bash would interpret in the same way. Thus far, I've come up with this:
[1-3][0-9]/[A-Z][a-z]\{2\}/(19|20)[0-9]\{2\}:[0-2][0-9]:[0-5][0-9]:[0-5][0-9]
The full command I'm using is
expr "$line" : '\([1-3][0-9]/[A-Z][a-z]\{2\}/(19|20)[0-9]\{2\}:[0-2][0-9]:[0-5][0-9]:[0-5][0-9]\)'
where $line contains the string out of which I need to extract the date. Unfortunately my bash version of the regex doesn't work. I have tried different things, like escaping / and :, but I can't seem to get it to work. What am I doing wrong?

Answer 1

The only problem was your first pattern [1-3]. It should be [0-3].
[[ $DATE =~ [0-3][0-9]/[A-Z][a-z]{2}/(19|20)[0-9]{2}:[0-9]{2}:[0-5][0-9]:[0-5][0-9] ]]
Also, on some earlier versions of Bash you have to store it on a variable:
RE='[0-3][0-9]/[A-Z][a-z]{2}/(19|20)[0-9]{2}:[0-9]{2}:[0-5][0-9]:[0-5][0-9]'
[[ $DATE =~ $RE ]]
Example:
> DATE='01/Jan/2000:23:59:59'
> [[ $DATE =~ [0-3][0-9]/[A-Z][a-z]{2}/(19|20)[0-9]{2}:[0-9]{2}:[0-5][0-9]:[0-5][0-9] ]] && echo Match.
Match.
Bash 3.0:
> echo "$BASH_VERSION"
3.00.0(1)-release
> DATE='01/Jan/2000:23:59:59'
> RE='[0-3][0-9]/[A-Z][a-z]{2}/(19|20)[0-9]{2}:[0-9]{2}:[0-5][0-9]:[0-5][0-9]'
> [[ $DATE =~ $RE ]] && echo Match.
Match.
If you want to apply it on a loop, you can have something like this:
RE='[0-3][0-9]/[A-Z][a-z]{2}/(19|20)[0-9]{2}:[0-9]{2}:[0-5][0-9]:[0-5][0-9]'
while read -r LINE; do
[[ $LINE =~ $RE ]] && echo "Match: $LINE"
done < date_list.txt
By the way, if you want to exactly match the whole word only use add ^ and $ at the beginning and the end of pattern:
[[ $DATE =~ ^[0-3][0-9]/[A-Z][a-z]{2}/(19|20)[0-9]{2}:[0-9]{2}:[0-5][0-9]:[0-5][0-9]$ ]]
To extract matches on the line use () and BASH_REMATCH:
[[ $DATE =~ .*([0-3][0-9]/[A-Z][a-z]{2}/(19|20)[0-9]{2}:[0-9]{2}:[0-5][0-9]:[0-5][0-9]).* ]] && echo "${BASH_REMATCH[1]}"