Using sed to modify an undefined string

Question

I need to use sed to modify a document called "case" with the lines example="value" where value can take any 2 o 3 characters integer, for example:

example=55
example=777
otherthing=anothervalue

I want to use sed to modify both of the example lines into two lines called example=000

I have tried to use

sed -i -e "s/example=*/example=000/g" case 

to modify them both at the same time, but obviously it did not worked. What I need to know is how to make sed take any 2 or 3 characters after the =

Answer 1

You're confusing the different meanings of * for Shell Filename Expansion and Posix Basic Regex (BRE).

In Regex, * is the quantifier for the character in front of it, so =* means 0 or more occurrences of =. If you want "any number of any character", use .*.

sed -i -e 's/example=.*/example=000/' case 

Some more hints:

• You can leave out the g modifier, as your string likely appears only once per line.
• If you don't want to also match another_example=value, use ^example=.* to make sure that example is at the start of the line.
• Instead of . for any character, you can use [0-9] or [[:digit:]]] to match only numbers.
• Instead of *, you can use + quantifier, that means 1 or more characters (must be escaped to \+ for BRE in GNU sed or written as \{1,\} for general sed).

---

To match only 2 or 3 characters, you can do

sed -i -e 's/example=.\{2,3\}/example=000/' case 

or using Extended Regex (ERE) (sed -E):

sed -i -E -e 's/example=.{2,3}/example=000/' case