Pattern syntax : pattern « XML Schema « XML Tutorial

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2. XML Schema
3. pattern

.          for any character at all
\d         for any digit; 
\D         for any non-digit
\s         for any white space (including space, tab, newline, and return); 
\S         for any character that is not white space
x*         to have zero or more x's; 
(xy)*      to have zero or more xy's
x?         to have one or zero x's; 
(xy)?      to have one or no xy's
x+         to have one or more x's; (xy)+ to have one or more xy's
[abc]      to include one of a group of values (a, b, or c)
[09]      to include the range of values from 0 to 9
A|B        to have A or B in the content. 
x{5}       to have exactly 5 x's (in a row)
x{5,}      to have at least 5 x's (in a row)
x{5,8}     to have at least 5 and at most 8 x's (in a row)
(xyz){2}   to have exactly two xyz's (in a row).