[antlr-interest] Matching empty string

[Dukie Banderjee]

Hi,
My grammar needs to handle the following situation: A line can have multiple fields, separated by a delimiter. A field can have multiple components, separated by another delimiter.
If a field or component is blank, it should be counted as a blank field or blank component. For example with field delimiter '+' and component delimiter ':' :
UNB++::+123
is a 'UNB' line with 3 fields. The first field is blank, the second field has 3 blank components, and the last field has a single component with the value '123'.

Here is my grammar so far:

line         : TEXT fields ;
fields        : field* terminator! ;
field        : SEP t=fieldText? -> ^(FIELD $t?) ;
fieldText    : comp (CSEP comp)* ;
comp        : t=TEXT -> ^(COMP $t) ;

When a field is blank, e.g. '++', this correctly generates a ^(FIELD) with no children. However, I don't know how to get similar behaviour for the components, because whereas a field starts with a SEP and optional TEXT, the component may or may not have a leading CSEP. 

When the input is '+::+', there are three components, but the first is entirely blank, an empty string. 

What I would like is that '+::+' creates ^(FIELD ^(COMP) ^(COMP) ^(COMP)). How can I accomplish this?

Thanks,

Rob

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