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 3.6 Basic vs Extended Regular Expressions
 =========================================
 
 Basic regular expressions differ from extended regular expressions in
 the following ways:
 
    • The characters ‘?’, ‘+’, ‘{’, ‘|’, ‘(’, and ‘)’ lose their special
      meaning; instead use the backslashed versions ‘\?’, ‘\+’, ‘\{’,
      ‘\|’, ‘\(’, and ‘\)’.  Also, a backslash is needed before an
      interval expression's closing ‘}’.
 
    • An unmatched ‘\)’ is invalid.
 
    • If an unescaped ‘^’ appears neither first, nor directly after ‘\(’
      or ‘\|’, it is treated like an ordinary character and is not an
      anchor.
 
    • If an unescaped ‘$’ appears neither last, nor directly before ‘\|’
      or ‘\)’, it is treated like an ordinary character and is not an
      anchor.
 
    • If an unescaped ‘*’ appears first, or appears directly after ‘\(’
      or ‘\|’ or anchoring ‘^’, it is treated like an ordinary character
      and is not a repetition operator.
 

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