Hello, in the section https://www.gnu.org/software/bash/manual/bash.html#Conditional-Constructs you have this example: [[ $line =~ [[:space:]]*?(a)b ]] The expression ?() is not part of the Posix ERE. It is part of bash pattern matching: https://www.gnu.org/software/bash/manual/bash.html#Pattern-Matching Consider this example, which will not match: line="aaab" echo $line if [[ $line =~ '?(a)b' ]]; then echo Match echo ${BASH_REMATCH[0]} echo ${BASH_REMATCH[1]} else echo No match fi Kind regards, Roland
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