File: grep.info,  Node: Basic vs Extended,  Next: Problematic Expressions,  Prev: Back-references and Subexpressions,  Up: Regular Expressions

3.6 Basic vs Extended Regular Expressions
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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.