SGML (Sounds like sigmal)

sample1.txt

<OUTER>
    <GROUP1>
        <TAG1>some data.
        <TAG2>SOME_MORE_DATA
        <TAG3>
            <TAG3A>test data
        </TAG3>
        <TAG4>FINAL TAG!
    </GROUP1>
</OUTER>

sample2.txt

<OUTER><GROUP1><TAG1>some data.<TAG2>SOME_MORE_DATA<TAG3><TAG3A>test/data</TAG3><TAG4>FINAL TAG!@#$%^&*()_+-=;':"[]{}\|,./?`~</GROUP1></OUTER>

sample3.txt

<OUTER>
    <GROUP1>
        <TAG1>
        <TAG2>SOME_MORE_DATA
        <TAG3>
            <TAG3A>test/data
        </TAG3>
        <TAG4>FINAL TAG!@#$%^&*()_+-=;':"[]{}\|,./?`~
    </GROUP1>
</OUTER>

sample4.txt

<OUTER>
	<TAG1>data
	<TAG2>data
	<TAG3>data
</OUTER>

sample5.txt

<OUTER>
    <TAG1>data
	<GROUP1>
		<TAG2>somedata!@#$%^&*()_+-=~`[]{},.?|\:;'"
		<TAG3>some datA
		<GROUP2>
			<TAG4>SOME_MORE_DATA
		</GROUP2>
	</GROUP1>
</OUTER>

SGML.g4

grammar SGML;

// rules
root       : aggregate ;
aggregate  : START_TAG ( aggregate | element )* END_TAG ;
element    : START_TAG content END_TAG? ;
content    : WORD+ ;

// tokens
START_TAG  : LT IDENTIFIER GT ;
END_TAG    : LT SLASH IDENTIFIER GT ;
WORD       : ( ALPHA | DIGIT | SYMBOL )+ ;
IDENTIFIER : UPPER_ALPHA ( UPPER_ALPHA | DIGIT )* ;

// skip tokens
WHITESPACE : ( SPACE | TAB )+ -> skip ;
NEWLINE    : [\r\n]+ -> skip ;

// building blocks
fragment LOWER_ALPHA   : [a-z] ;
fragment UPPER_ALPHA   : [A-Z] ;
fragment ALPHA         : [a-zA-Z] ;
fragment DIGIT         : [0-9] ;
fragment SYMBOL        : [.!@#$%^&*()_+-=~`,?\\\[\]{}|:;'"] ;
fragment SPACE         : ' ' ;
fragment TAB           : [\t] ;
fragment LT            : '<' ;
fragment GT            : '>' ;
fragment SLASH         : '/' ;