问题内容: Qustion What is the reason that POSIX expression such as [:space:] needs to be in another [ ] ? $ echo "a b c" | sed 's/[:space:]*/_/g' _ _b_ _ $ echo "a b c" | sed 's/[[:space:]]*/_/g' _a_b_c_ $ echo "a b c" | sed 's/[[:space:]][[:space:]]*/_/ […]