find: posix-awk regular expression syntax

1 
1 8.5.7 'posix-awk' regular expression syntax
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1 
1 The character '.' matches any single character except the null
1 character.
1 
1 '+'
1      indicates that the regular expression should match one or more
1      occurrences of the previous atom or regexp.
1 '?'
1      indicates that the regular expression should match zero or one
1      occurrence of the previous atom or regexp.
1 '\+'
1      matches a '+'
1 '\?'
1      matches a '?'.
1 
1    Bracket expressions are used to match ranges of characters.  Bracket
1 expressions where the range is backward, for example '[z-a]', are
1 invalid.  Within square brackets, '\' can be used to quote the following
1 character.  Character classes are supported; for example '[[:digit:]]'
1 will match a single decimal digit.
1 
1    GNU extensions are not supported and so '\w', '\W', '\<', '\>', '\b',
1 '\B', '\`', and '\'' match 'w', 'W', '<', '>', 'b', 'B', '`', and '''
1 respectively.
1 
1    Grouping is performed with parentheses '()'.  An unmatched ')'
1 matches just itself.  A backslash followed by a digit acts as a
1 back-reference and matches the same thing as the previous grouped
1 expression indicated by that number.  For example '\2' matches the
1 second group expression.  The order of group expressions is determined
1 by the position of their opening parenthesis '('.
1 
1    The alternation operator is '|'.
1 
1    The characters '^' and '$' always represent the beginning and end of
1 a string respectively, except within square brackets.  Within brackets,
1 '^' can be used to invert the membership of the character class being
1 specified.
1 
1    '*', '+' and '?' are special at any point in a regular expression
1 except the following places, where they are not allowed:
1 
1   1. At the beginning of a regular expression
1 
1   2. After an open-group, signified by '('
1   3. After the alternation operator '|'
1 
1    Intervals are specified by '{' and '}'.  Invalid intervals are
1 treated as literals, for example 'a{1' is treated as 'a\{1'
1 
1    The longest possible match is returned; this applies to the regular
1 expression as a whole and (subject to this constraint) to subexpressions
1 within groups.
1