Using Regular Expressions
-------------------------

A regular expression is a string of text that contains
symbolic characters representing various patterns the
user wants to find. It is very similar to a mathematical
expression.

Regular expressions are built from the following elements:

.          Matches any single character.
[ ]        Matches any one of the characters in the
           brackets, or any of a range of characters
           separated by a hyphen '-', or a character class
           operator (see below).
[^]        Matches any characters except for those after the
           caret '^'.
^          Anchors matching to the start of a line.
$          Anchors matching to the end of a line.
( )        Encloses a subexpression.
*          Matches 0 or more of what precedes.
?          Matches 0 or 1 of what precedes.
+          Matches 1 or more of what precedes.
{count}    Matches count of what precedes.
{min,}     Matches at least min of what precedes.
{min,max}  Matches between min and max of what precedes.
|          Matches either what is to the left or right.

\.         Matches '.'.
\[         Matches '['.
\]         Matches ']'.
\^         Matches '^'.
\$         Matches '$'.
\(         Matches '('.
\)         Matches ')'.
\*         Matches '*'.
\?         Matches '?'.
\+         Matches '+'.
\{         Matches '{'.
\}         Matches '}'.
\|         Matches '|'.
\\         Matches '\'.

Character Class Operators
-------------------------

[:alnum:]   The set of alpha-numeric characters.
[:alpha:]   The set of alphabetic characters.
[:blank:]   Tab and space.
[:cntrl:]   The control characters.
[:digit:]   Decimal digits.
[:graph:]   All printable characters except space.
[:lower:]   Lower case letters.
[:print:]   The "printable" characters.
[:punct:]   Punctuation.
[:space:]   Whitespace characters.
[:upper:]   Upper case letters.
[:xdigit:]  Hexadecimal digits.

Examples
--------

abcde       Matches the characters 'a', 'b', 'c', 'd', and
            'e' in contiguous order.
d.n         Matches three characters starting with a 'd' and
            ending with an 'n'.
[abcde]     Matches any one of the characters 'a', 'b', 'c',
            'd', or 'e'.
[a-e]       Matches any one of the characters 'a', 'b', 'c',
            'd', or 'e'.
[A-Za-z]    Matches any single letter.
re[aei]d    Matches 'read', 'reed', or 'reid'.
[^a-e]      Matches any character except one of 'a', 'b',
            'c', 'd', or 'e'.
h[^u]t      Matches three characters starting with 'h' and
            ending with 't', except 'hut'.
^start      Matches 'start' when it is the first word on a
            line.
end$        Matches 'end' when it is the last word on a
            line.
^$          Matches a blank line (start, nothing, and end).
^[0-9]$     Matches a line containing only one digit.
ho*p        Matches 'hp', 'hop', 'hoop', and so on.
ho?p        Matches 'hp' and 'hop', but not 'hoop'.
ho+p        Matches 'hop', 'hoop', and so on, but not 'hp'.
[0-9]+      Matches an integer (one or more digits).
ho{2}p      Matches 'hoop', but not 'hop'.
ho{1,}p     Matches 'hop', 'hoop', and so on, but not 'hp'.
ho{1,2}p    Matches 'hop' and 'hoop', but not 'hp' or
            'hooop'.
[2-9][0-9][0-9]-[0-9]{4}
            Matches a North American Dialing Zone phone
            number; i.e., a digit between 2 and 9, two
            digits, a hyphen and four digits.
hop|hoop    Matches 'hop' or 'hoop'.
(Sam|Mary|Bill) (Smith|Jones)
            Matches any one of the six names 'Sam Smith',
            'Sam Jones', 'Mary Smith', 'Mary Jones',
            'Bill Smith' or 'Bill Jones'.
\[\]\\      Matches the characters '[', ']', and '\' in
            contiguous order.

Using Replacement Expressions
-----------------------------

In both regular and replacement expressions, the following
special characters are defined as:

\n    The newline character.
\t    The tab character.
\\    The backslash character.

The following elements are used in regular expressions to
match, or in replacement expressions to replace,
subexpressions enclosed by '(' and ')'. Subexpressions are
numbered from left to right:

\0    The entire expression.
\1    The 1st subexpression.
\2    The 2nd subexpression.
\3    The 3rd subexpression.
\4    The 4th subexpression.
\5    The 5th subexpression.
\6    The 6th subexpression.
\7    The 7th subexpression.
\8    The 8th subexpression.
\9    The 9th subexpression.

Examples
--------

the regular expression:           (word) \1 \1
matches:                          word word word

and

the regular expression:           (first) (second) (third)
and the replacement expression:   \3 \2 \1
reverses the order of the words:  first second third
to become:                        third second first