How to use grep with a pipe symbol from the command line

Question

Maybe I'm missing something very simple, but I don't seem to be able to use grep from the command line (in bash) where the search argument contains a vertical bar |.
For example,

grep a|b

results in the error bash: b: command not found. Of course.
Escaping the pipe

grep a\|b

gets rid of the error, but results in grep itself interpreting the pipe as a literal search character (i.e. it searches for the text a|b in the input rather than "a or b").

grep "a|b"

has the same result: it also searches for the literal text a|b.

So, what now? I also tried ' and ` for delimiters, and now I'm out of things to think of.
Open to suggestions!

(In case it matters, this is with Debian 12, KDE and konsole for a terminal.)

Answer 1

gets rid of the error, but results in grep itself interpreting the pipe as a literal search character (i.e. it searches for the text a|b in the input rather than "a or b").

It has no effect on grep interpreting the pipe (the kind of quoting you used is invisible to grep; it only receives the string a|b either way) – grep always interprets pipe as a literal character.

Grep uses POSIX 'basic' regular expressions, which don't have this functionality at all (although some implementations add it using \|).

Use grep -E or egrep to activate the "POSIX extended" regular expression language where | actually means what you expect it to mean, or grep -P to activate support for PCRE (Perl-like) regular expressions.