===========================
        DATA
        
        
patternAB = [Sequence, 
            [[ZeroOrMore, "a"], 
            [Text, "b"]]].

==========================
        HANDLERS

Matcher = type variant (Sequence, ZeroOrMore, Text)

matchSequence = function(text::string, sequence::undef):: int{
    textLength = length(text):: int.
    
    loop fold(sequence-> matcher:: undef, 0->pos){
        n = match(mid(text, pos, textLength), matcher).
        
        if (n == pos_FAIL:: break){
            pos
            
        } else {
            pos+n
        }
    }
}

matchZeroOrMore= function(text::string, matcher::string):: int{   
    textLength = length(text):: int.
    
    loop fold inf(0->pos, n==pos_FAIL){                              //<---         infinite loop?
        n = match(matcherChild, mid(text, pos, textLength)).
        
        if (n == pos_FAIL:: break){
            pos
            
        } else {
            pos+n
        }
    }
}

matchText = function(text::string, matcher::string):: int {   
    textLength = length(text):: int.
    
    if (length(matcher) <= textLength, mid(text, 0, length(matcher)) == matcher)
    {
        textLength;
        
    } else {
        pos_FAIL;
    }
}

match = function(text::string, pattern::undef)::string{

    n = switch (pattern[0]) :: int
    case (Sequence) {matchSequence(text, pattern[1])}
    case (ZeroOrMore) {matchZeroOrMore(text, pattern[1])}
    case (Text) {matchText(text, pattern[1])}
    case default {pos_FAIL}
        
    if (n != pos_FAIL)
        {mid(text, 0, n)}
    else 
        {text_FAIL}
}

=================================
    CLIENT
    
test = function(){
    match(patternAB, "aaaaab").
    match(patternAB, "baaaaa").
}