<label>${Name}</label>

<?xml version="1.0" encoding="UTF-8"?>
<extension xmlns="http://www.eclipse.org/sapphire/xmlns/extension" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    <function>
        <name>x:Substring</name>
        <impl>MY.COMPANY.PATH.el.SubstringFunction</impl>
    </function>
</extension>

import org.eclipse.sapphire.modeling.el.Function;
import org.eclipse.sapphire.modeling.el.FunctionContext;
import org.eclipse.sapphire.modeling.el.FunctionResult;

public final class SubstringFunction extends Function {
    @Override
    public String name() {
        return "x:Substring";
    }

    @Override
    public FunctionResult evaluate(final FunctionContext context) {
        return new FunctionResult(this, context) {
            @Override
            protected Object evaluate() {
                String value = cast(operand(0).value(), String.class);
                Integer startIndex = cast(operand(1).value(), Integer.class);
                Integer endIndex = cast(operand(2).value(), Integer.class);

                if (value == null || value.isEmpty() || startIndex.equals(endIndex)) {
                    return "";
                }

                if (value.length() <= (startIndex + endIndex)) {
                    return value;
                }

                return value.substring(startIndex, endIndex);
            }
        };
    }

}

<label>${x:Substring(Name,0,20)}</label>