Matching optional parameters with non-capturing groups in Bash regular expression

I don't think non-capturing groups exist in bash regex, so your options are to use a scripting language or to remove the ?: from all of the (?:...) groups and just be careful about which groups you reference, for example:

CATEGORY_REGEX='Category:\s*([^;]*);\s*scheme="([^"]*)";\s*class="([^"]*)";\s*(title="([^"]*)";)?\s*(rel="([^"]*)";)?\s*(location="([^"]*)";)?\s*(attributes="([^"]*)";)?\s*(actions="([^"]*)";)?'
category_string='Category: entity;scheme="http://schemas.ogf.org/occi/core#";class="kind";title="Entity";attributes="occi.core.id occi.core.title";'
[[ $category_string =~ $CATEGORY_REGEX ]]
echo "full:       ${BASH_REMATCH[0]}"
echo "category:   ${BASH_REMATCH[1]}"
echo "scheme:     ${BASH_REMATCH[2]}"
echo "class:      ${BASH_REMATCH[3]}"
echo "title:      ${BASH_REMATCH[5]}"
echo "rel:        ${BASH_REMATCH[7]}"
echo "location:   ${BASH_REMATCH[9]}"
echo "attributes: ${BASH_REMATCH[11]}"
echo "actions:    ${BASH_REMATCH[13]}"

Note that starting with the optional parameters we need to skip a group each time, because the even numbered groups from 4 on contain the parameter name as well as the value (if the parameter is present).