Using regex comparision in bash and BASH_REMATCH

Bash supports regular expressions in comparisons via the =~ operator. But what is rarely used or documented is that you can use the ${BASH_REMATCH[n]} array to access successful matches (back-references to capture groups). So if you use parentheses for grouping () in your regex, you can access the content of that group.

Here is an example where I am parsing date placeholders in a text with an optional offset (e.g. |YYYY.MM.DD|+2 ). Storing the format and offset in separate groups:

while read -r line; do
	while [[ ${line} =~ \|([YMD\\/\ .-]+)\|(\+*[0-9]*) ]]; do
		dateformat=${BASH_REMATCH[1]}
		dateformat=${dateformat/YYYY/%Y}
		dateformat=${dateformat/MMMM/%B}
		dateformat=${dateformat/MM/%m}
		dateformat=${dateformat/DD/%d}
		offset='now'
		[[ ! -z ${BASH_REMATCH[2]} ]] && offset="${BASH_REMATCH[2]} days"
		line=${line/|${BASH_REMATCH[1]}|${BASH_REMATCH[2]}/$(date "+${dateformat}" --date="${offset}")}
	done
	echo "${line}"
done < input

while read -r line; do

while [[ ${line} =~ \|([YMD\\/\ .-]+)\|(\+*[0-9]*) ]]; do

dateformat=${BASH_REMATCH[1]}

dateformat=${dateformat/YYYY/%Y}

dateformat=${dateformat/MMMM/%B}

dateformat=${dateformat/MM/%m}

dateformat=${dateformat/DD/%d}

offset='now'

[[ ! -z ${BASH_REMATCH[2]} ]] && offset="${BASH_REMATCH[2]} days"

line=${line/|${BASH_REMATCH[1]}|${BASH_REMATCH[2]}/$(date "+${dateformat}" --date="${offset}")}

done

echo "${line}"

done < input

|YYYY.MM.DD|

|YYYY.MM.DD|+7

|YYYY-MM-DD|

|YYYY-MM-DD|+14

|MMMM YYYY|

|YYYY/MM|

|MM/YYYY|

This is a sentence containing a timestamp (|YYYY.MM.DD|+7) with an offset.

This is another sentence containing multiple timstamps between |YYYY.MM.DD| and |YYYY.MM.DD|+7.

2017.09.08

2017.09.15

2017-09-08

2017-09-22

September 2017

2017/09

09/2017

This is a sentence containing a timestamp (2017.09.15) with an offset.

This is another sentence containing multiple timstamps between 2017.09.08 and 2017.09.15.