Using double quotes

Use double quotes to make the shell expand variables while preserving whitespace:

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sed -i "s/$var1/ZZ/g" "$file"

When you require the quote character in the replacement string, you have to precede it with a backslash which will be interpreted by the shell. In the following example, the string quote me will be replaced by "quote me" (the character & is interpreted by sed):

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sed -i "s/quote me/\"&\"/" "$file"

Using single quotes

If you've a lot shell meta-characters, consider using single quotes for the pattern, and double quotes for the variable:

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sed -i 's,'"$pattern"',Say hurrah to &: \0/,' "$file"