find: egrep regular expression syntax

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1 8.5.3 'egrep' regular expression syntax
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1 The character '.' matches any single character.
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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, '\' is taken literally.  Character
1 classes are supported; for example '[[:digit:]]' will match a single
1 decimal digit.
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1    GNU extensions are supported:
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1   1. '\w' matches a character within a word
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1   2. '\W' matches a character which is not within a word
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1   3. '\<' matches the beginning of a word
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1   4. '\>' matches the end of a word
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1   5. '\b' matches a word boundary
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1   6. '\B' matches characters which are not a word boundary
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1   7. '\`' matches the beginning of the whole input
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1   8. '\'' matches the end of the whole input
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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 '('.
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1    The alternation operator is '|'.
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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.
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1    The characters '*', '+' and '?' are special anywhere in a regular
1 expression.
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1    Intervals are specified by '{' and '}'.  Invalid intervals are
1 treated as literals, for example 'a{1' is treated as 'a\{1'
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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.
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