Replace all non-alphanumeric characters in a string with an underscore

Question

I want to replace special characters (regex \W) with _ (underscore) But I don't want to replace whitespace with underscore Also replace multiple consecutive special characters with single underscore

Example String: The/Sun is red@ Output: The_Sun is red_

String: .//hack Moon Output: _hack Moon

I have tried echo 'string' | sed 's/\W/_/g' But it's not accurate

Answer 1

sed approach:

s="The/Sun is red@ .//hack Moon"

sed -E 's/[^[:alnum:][:space:]]+/_/g' <<<"$s"
The_Sun is red_ _hack Moon

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• [^[:alnum:][:space:]]+ - match any character sequence except alphanumeric and whitespace

Answer 2

Use tr for that:

echo "The/Sun is red@" | tr -s -c [:alnum:][:blank:] _

[:alnum:][:blank:] represents alphanumeric characters and whitespace, -c means the opposite of that.

Added: -s to squeeze duplicate underscores into one.

Answer 3

Just with bash parameter expansion, similar pattern to other answers:

shopt -s extglob
for str in "The/Sun is red@" ".//hack Moon"; do 
    echo "${str//+([^[:alnum:][:blank:]])/_}"
    # .........^^........................^  replace all
    # ...........^^.....................^    one or more
    # .............^^^^^^^^^^^^^^^^^^^^^      non-alnum, non-space character
done

The_Sun is red_
_hack Moon