Regular Expression Compiler

sub at_start { '^'; }
sub at_end { '$'; }
sub any { "."; }
sub zero_or_more { "(?:$_[0])*"; }
sub one_or_more { "(?:$_[0])+"; }
sub optional { "(?:$_[0])?"; }
sub remember { "($_[0])"; }
sub one_of { "(?:" . join("|", @_) . ")"; }
sub in_charset { "[$_[0]]"; }       # I know it's broken for ']'...
sub not_in_charset { "[^$_[0]]"; }   # I know it's broken for ']'...

at_start .
'"' .
zero_or_more(
    one_of(
        not_in_charset('\\"'),    # Yuck, 2 levels of escaping required
        '\\' . any
    )
) .
'"'

c(a|d)+r

<sequence>
 <element name="c" type="xs:string"/>
 <choice minOccurs="1" maxOccurs="unbounded">
  <element name="a" type="xs:string"/>
  <element name="d" type="xs:string"/>
 </choice>
 <element name="r" type="xs:string"/>
<sequence>