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 A regular expression is a text pattern made up of a concatenation of the
 following elements:

     c        literal character c
     ?        any character except newline
     ^        beginning of line
     $        end of line (null string before newline)
     [...]    character class (any one of these characters)
     [~...]   negated character class (all but these characters)
     *        closure (zero or more occurrences of previous pattern)
     @c       escaped character (e.g. @^, @$, @[, @*,)

 Special meaning of characters in a text pattern is lost when escaped,
 inside [...] (except for @]), or for:

     ^        not at beginning of pattern
     $        not at end of pattern
     *        at beginning of pattern

 A character class consists of zero or more of the following elements,
 surrounded by [ and ]:

     c        literal character c, including [
     c1-c2    range of characters (digits, lower or upper case letters)
     ~        negated character class if at beginning
     @c       escaped characters (e.g. @~, @-, @@, @])

 Special meaning of characters in a character class is lost when escaped
 or for:

     ~        not at beginning
     -        at beginning or end

 An escape sequence consists of the character @ followed by a single

     @n       newline
     @N       newline
     @t       tab
     @T       tab
     @c       c (include @@)

 For example, to match all words beginning with a letter, followed by
 zero or more letters or digits, and ending with an "s", use the pattern: