Under zsh, looking to use `tr` to search and replace a single character with multiple characters

Question

I want to replace "Blah: whatever" or "Blah/Whatever" with "Blah--Whatever"

I have this:

NEW=`echo ${OLD} | tr '[:/]' '--'`
echo ${NEW}

I suppose it's taking the 2 dash characters to mean 1 letter replacement for : or /

How do i tell tr to replace : or / with "--"?

using zsh

Answer 1

tr only does character-for-character replacements. Try this:

NEW=${OLD//[:\/]/--}