Apparently, ctags applies the rules before deleting comments:
ctags: Warning: include/linux/completion.h:22: null expansion of name pattern "\2"
Work around this particular case by requiring the group to contain at
least one character. Leave the other patters as they are, until a better
solution is found.
Signed-off-by: Michal Marek <mmarek@suse.com>
        --regex-c='/PCI_OP_WRITE\((\w*).*[1-4]\)/pci_bus_write_config_\1/' \
        --regex-c='/DEFINE_(MUTEX|SEMAPHORE|SPINLOCK)\((\w*)/\2/v/'     \
        --regex-c='/DEFINE_(RAW_SPINLOCK|RWLOCK|SEQLOCK)\((\w*)/\2/v/'  \
-       --regex-c='/DECLARE_(RWSEM|COMPLETION)\((\w*)/\2/v/'            \
+       --regex-c='/DECLARE_(RWSEM|COMPLETION)\((\w+)/\2/v/'            \
        --regex-c='/DECLARE_BITMAP\((\w*)/\1/v/'                        \
        --regex-c='/(^|\s)(|L|H)LIST_HEAD\((\w*)/\3/v/'                 \
        --regex-c='/(^|\s)RADIX_TREE\((\w*)/\2/v/'                      \