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2.1 Command-line Options
grep comes with a rich set of options:
some from POSIX.2 and some being GNU extensions.
Long option names are always a GNU extension,
even for options that are from POSIX specifications.
Options that are specified by POSIX,
under their short names,
are explicitly marked as such
to facilitate POSIX-portable programming.
A few option names are provided
for compatibility with older or more exotic implementations.
Several additional options control
which variant of the grep matching engine is used.
See section grep Programs.