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What is the function of [0-9]* in the following statement, which will write all lines that start with an even number, followed by a space, to the file even:
sed -n 's/^[0-9]*[02468] /&/w even' <file
Then, eliminating [0-9]* will have the same function/output. In other words, both statements are the same effect.
sed -n 's/^[02468] /&/w even' <file
Is [0-9]* useless here? I am really confused. Please help me.
The [0-9]* is not useless; it matches any number (0-9) that show up before the even number range ([02468]). This is to take into account multi-digit even numbers.
For e.g., if you didn't have ^[0-9]* anchored to beginning of your pattern, it would not match:
92
910
308
20
The other pattern you mentioned (/^[02468]/) would only match anything that begins with an even digit.
