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I want to replace all symbols like '.','-','/' and so on by ' '

I triedsed 's/[[!:alnum:]]/ /g' example.txt but it does not work

and for sed 's/[,;\.\-\/()_]/ /g' example.txt, I found that I cannot replace '-' by ' '.

is there any way to do that?

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To match all non-alphanumerics, use [^[:alnum:]]. It's similar to [^abc]: the leading caret makes the bracket expression match all but the listed characters.

To match a literal dash, you need to make sure it's not interpreted as creating a list of characters to match. Make it the first character of the bracket expression: [-,.;/()_].

Note that sed doesn't take the / as a separator for s/// if it's within a bracket expression, so there's no need to put a backslash in front of it. (The backslash will match itself.) E.g. echo 'a\b/c' | sed 's/[/\]/./g' prints a.b.c.

See for example the regex(7) man page for a description of the regular expression syntax. (Note that it mainly discusses extended regular expressions (ERE), used by sed -E in many/most seds.)

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