How many positive integers less than $2019$ are divisible by either $18$ or $21$, but not both?
1. **Calculate the number of integers less than 2019 that are divisible by 18:**
   \[
   \left\lfloor \frac{2018}{18} \right\rfloor = \left\lfloor 112.1111 \right\rfloor = 112
   \]
   So, there are 112 positive integers less than 2019 that are divisible by 18.

2. **Calculate the number of integers less than 2019 that are divisible by 21:**
   \[
   \left\lfloor \frac{2018}{21} \right\rfloor = \left\lfloor 96.0952 \right\rfloor = 96
   \]
   So, there are 96 positive integers less than 2019 that are divisible by 21.

3. **Calculate the number of integers less than 2019 that are divisible by both 18 and 21:**
   - First, find the least common multiple (LCM) of 18 and 21:
     \[
     \text{LCM}(18, 21) = \frac{18 \times 21}{\text{GCD}(18, 21)}
     \]
     The greatest common divisor (GCD) of 18 and 21 is 3:
     \[
     \text{LCM}(18, 21) = \frac{18 \times 21}{3} = 126
     \]
   - Now, calculate the number of integers less than 2019 that are divisible by 126:
     \[
     \left\lfloor \frac{2018}{126} \right\rfloor = \left\lfloor 16.0159 \right\rfloor = 16
     \]
   So, there are 16 positive integers less than 2019 that are divisible by both 18 and 21.

4. **Apply the principle of inclusion-exclusion (PIE) to find the number of integers divisible by either 18 or 21:**
   \[
   112 + 96 - 16 = 192
   \]
   So, there are 192 positive integers less than 2019 that are divisible by either 18 or 21.

5. **Calculate the number of integers divisible by either 18 or 21, but not both:**
   - Subtract the number of integers divisible by both 18 and 21 from the total number of integers divisible by either 18 or 21:
     \[
     192 - 16 = 176
     \]
   So, there are 176 positive integers less than 2019 that are divisible by either 18 or 21, but not both.

The final answer is \(\boxed{176}\)