sed: ERE syntax

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1 5.4 Overview of extended regular expression syntax
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1 The only difference between basic and extended regular expressions is in
1 the behavior of a few characters: '?', '+', parentheses, braces ('{}'),
1 and '|'.  While basic regular expressions require these to be escaped if
1 you want them to behave as special characters, when using extended
1 regular expressions you must escape them if you want them _to match a
1 literal character_.  '|' is special here because '\|' is a GNU extension
1 - standard basic regular expressions do not provide its functionality.
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1 Examples:
1 'abc?'
1      becomes 'abc\?' when using extended regular expressions.  It
1      matches the literal string 'abc?'.
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1 'c\+'
1      becomes 'c+' when using extended regular expressions.  It matches
1      one or more 'c's.
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1 'a\{3,\}'
1      becomes 'a{3,}' when using extended regular expressions.  It
1      matches three or more 'a's.
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1 '\(abc\)\{2,3\}'
1      becomes '(abc){2,3}' when using extended regular expressions.  It
1      matches either 'abcabc' or 'abcabcabc'.
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1 '\(abc*\)\1'
1      becomes '(abc*)\1' when using extended regular expressions.
1      Backreferences must still be escaped when using extended regular
1      expressions.
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1 'a\|b'
1      becomes 'a|b' when using extended regular expressions.  It matches
1      'a' or 'b'.
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