protected ValueSet getValue(int v, String s, int i) {
        char c = s.charAt(i);
        StringBuilder s1 = new StringBuilder(String.valueOf(v));
        while (c >= '0' && c <= '9') {
            s1.append(c);
            i++;
            if (i >= s.length()) {
                break;
            }
            c = s.charAt(i);
        }
        ValueSet val = new ValueSet();

        val.pos = (i < s.length()) ? i : i + 1;
        val.value = Integer.parseInt(s1.toString());
        return val;
    }
Indicates whether the specified cron expression can be parsed into a
valid cron expression

@param cronExpression the expression to evaluate
@return a boolean indicating whether the given expression is a valid cron
        expression
lts-core/src/main/java/com/github/ltsopensource/core/support/CronExpression.java