FreeBSD/Linux

grep -E (same as `egrep', --extended-regexp)

ex)
grep -E '^(foo|bar)' file.txt

same as

grep '^\(foo\|boar\)'

In  basic  regular  expressions the metacharacters ?, +, {, |, (, and ) lose their special meaning;
instead use the  backslashed  versions  \?, \+, \{, \|, \(, and \).



-E, --extended-regexp
-G, --basic-regexp
-i, --ignore-case
-q, --quiet, --silent

-n, --line-number          /*** good idea ***/
      Prefix each line of output with the line number within its
      input file.

-r, --recursive
      Read all files under each directory, recursively; this is
      equivalent to the -d recurse option.

-v, --invert-match

-x, --line-regexp
      Select only those matches that exactly match the whole line.


[:alnum:],
[:alpha:],
[:cntrl:],
[:digit:],
[:graph:],
[:lower:],
[:print:],
[:punct:],
[:space:],
[:upper:],
[:xdigit:]

For example, [[:alnum:]]  means [0-9A-Za-z]

A regular expression may be followed by one of several repetition
operators:
  ?      The preceding item is optional and matched at most once.
  *      The preceding item will be matched zero or more times.
  +      The preceding item will be matched one or more times.
  {n}    The preceding item is matched exactly n times.
  {n,}   The preceding item is matched n or more times.
  {n,m}  The preceding item is matched at least n  times,  but
         not  more than m times.


EOF