为什么POSIX“可打印字符”类与简单字符串不匹配？

Question

I wrote the following script to test the "printable characters" character class, as described here .

#!/bin/sh

case "foo" in
    *[:print:]*) echo "found a printable character" ;;
    *) echo "found no printable characters" ;;
esac

I expect this script to output found a printable character , at least one (in fact, all) characters in "foo" are printable. Instead, it outputs "found no printable characters" . Why are the characters in "foo" not recognized as printable characters?

Answer 1

The string [: is only special inside a bracket expression and bracket expressions are themselves introduced by [ . So your example should be:

case "foo" in
    *[[:print:]]*) echo "found a printable character" ;;
    *) echo "found no printable characters" ;;
esac

If this seems cumbersome, think about for example how you would specify a pattern which should match a lowercase letter or a digit but not an upper case letter.

For more information see the section of the POSIX spec detailing bracket expressions in regular expressions . Bracket expressions in shell patterns are like bracket expressions in regular expressions, except for the treatment of ! and ^ . (Though otherwise there are other differences between shell patterns and regexes, outside the context of bracket expressions).