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Why grep ‘foo|bar’ file doesn’t work?

The bar | has no special meaning in BRE (basic regular expressions). Use extended regular expressions (ERE) such as:

grep -E 'foo|bar' file

or

egrep 'foo|bar' file

In GNU grep, you can also force the spcial meaning of | by escaping it. E.g.,

grep 'foo\|bar' file

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