Regex validation for pattern 20+99

Hi my requirement is like this

Required text format 20+99

1. maximum allowed number should be 20 for left side from the + mark
2. right side maximum number should be 99
3. should allow only two characters each side

Other allowed formats

1. 1+1

2. 0+0

3. 01+01

We tried followings

[0-9]{1,2}([+])[0-9]{1,2}

(^[01][0-9]|20)([+])[0-9]{1,2}

Problem with first approach is cannot limit left side to 20 and it allows more than 2 characters each side.

Problem with the second approach is it does not allow 1+1 or 0+0. Appreciate some support to modify the regex to cater our requirement.