grep searches a file for a given pattern.  Execute by:

   grep [flags] regular_expression file_list

Flags are single characters preceeded by '-':
   -c      Only a count of matching lines is printed
   -f      Print file name for matching lines switch, see below
   -n      Each line is preceeded by its line number
   -v      Only print non-matching lines

The file_list is a list of files.
The file name is normally printed if there is a file given.
The -f flag reverses this action.

The regular_expression defines the pattern to search for.  Upper- and
lower-case are always ignored.  Blank lines never match.  The expression
should be quoted to prevent file-name translation.

x      An ordinary character (not mentioned below) matches that character.

'\'    The backslash quotes any character. e.g. "\$" matches a dollar-sign.

'^'    A circumflex at the beginning of an expression matches the
       beginning of a line.

'$'    A dollar-sign at the end of an expression matches the end of a line.

'.'    A period matches any character except "new-line".

':a'   A colon matches a class of characters described by the following
':d'     character.  ":a" matches any alphabetic, ":d" matches digits,
':n'     ":n" matches alphanumerics, ": " matches spaces, tabs, and
': '     other control characters, such as new-line.

'*'    An expression followed by an asterisk matches zero or more
       occurrances of that expression: "fo*" matches "f", "fo", "foo", etc.

'+'    An expression followed by a plus sign matches one or more
       occurrances of that expression: "fo+" matches "fo", etc.

'-'    An expression followed by a minus sign optionally matches
       the expression.

'[]'   A string enclosed in square brackets matches any character in
       that string, but no others.  If the first character in the
       string is a circumflex, the expression matches any character
       except "new-line" and the characters in the string.  For
       example, "[xyz]" matches "xx" and "zyx", while "[^xyz]"
       matches "abc" but not "axb".  A range of characters may be
       specified by two characters separated by "-".  Note that,
       [a-z] matches alphabetics, while [z-a] never matches.

The concatenation of regular expressions is a regular expression.