/normal regex/iomx[neus]
%r|alternate form|

# options:

/i         case insensitive
/o         only interpolate #{} blocks once
/m         multiline mode - '.' will match newline
/x         extended mode - whitespace is ignored
/[neus]    encoding: none, EUC, UTF-8, SJIS, respectively

# regex characters:

.          any character except newline
[ ]        any single character of set
[^ ]       any single character NOT of set
*          0 or more previous regular expression
*?         0 or more previous regular expression(non greedy)
+          1 or more previous regular expression
+?         1 or more previous regular expression(non greedy)
?          0 or 1 previous regular expression
|          alternation
( )        grouping regular expressions
^          beginning of a line or string
$          end of a line or string
{m,n}      at least m but most n previous regular expression
{m,n}?     at least m but most n previous regular expression(non greedy)
\A         beginning of a string
\b         backspace(0x08)(inside[]only)
\b         word boundary(outside[]only)
\B         non-word boundary
\d         digit, same as[0-9]
\D         non-digit
\S         non-whitespace character
\s         whitespace character[ \t\n\r\f]
\W         non-word character
\w         word character[0-9A-Za-z_]
\z         end of a string
\Z         end of a string, or before newline at the end
(?# )      comment
(?: )      grouping without backreferences
(?= )      zero-width positive look-ahead assertion
(?! )      zero-width negative look-ahead assertion
(?ix-ix)   turns on/off i/x options, localized in group if any.
(?ix-ix: ) turns on/off i/x options, localized in non-capturing group.

# special character classes:

[:alnum:]   alpha-numeric characters
[:alpha:]   alphabetic characters
[:blank:]   whitespace - does not include tabs, carriage returns, etc
[:cntrl:]   control characters
[:digit:]   decimal digits
[:graph:]   graph characters
[:lower:]   lower case characters
[:print:]   printable characters
[:punct:]   punctuation characters
[:space:]   whitespace, including tabs, carriage returns, etc
[:upper:]   upper case characters
[:xdigit:]  hexadecimal digits

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