sed: BRE vs ERE

1 
1 5.2 Basic (BRE) and extended (ERE) regular expression
1 =====================================================
1 
1 Basic and extended regular expressions are two variations on the syntax
1 of the specified pattern.  Basic Regular Expression (BRE) syntax is the
1 default in 'sed' (and similarly in 'grep').  Use the POSIX-specified
1 '-E' option ('-r', '--regexp-extended') to enable Extended Regular
1 Expression (ERE) syntax.
1 
1    In GNU 'sed', the only difference between basic and extended regular
1 expressions is in the behavior of a few special characters: '?', '+',
1 parentheses, braces ('{}'), and '|'.
1 
1    With basic (BRE) syntax, these characters do not have special meaning
1 unless prefixed backslash ('\'); While with extended (ERE) syntax it is
1 reversed: these characters are special unless they are prefixed with
1 backslash ('\').
1 
1 Desired pattern      Basic (BRE) Syntax         Extended (ERE) Syntax
1                                                 
1 --------------------------------------------------------------------------
1 literal '+' (plus         $ echo 'a+b=c' > foo       $ echo 'a+b=c' > foo
1 sign)                     $ sed -n '/a+b/p' foo      $ sed -E -n '/a\+b/p' foo
1                           a+b=c                      a+b=c
1                                                 
1 One or more 'a'           $ echo aab > foo           $ echo aab > foo
1 characters                $ sed -n '/a\+b/p' foo     $ sed -E -n '/a+b/p' foo
1 followed by 'b'           aab                        aab
1 (plus sign as                                   
1 special
1 meta-character)
1