Hi there. Perhaps this will help! https://homeguides.sfgate.com/kill-crab-grass-organically-67475.html
Vinegar is what I used, but there are other methods. I can't use chemicals as I have a lot of pets and visiting children.
Dig,dig,dig! Take small shovel fulls and pick out every last root. If the patch isn't too large, I'd try boiling water before digging. It'll help kill the green blades in case you miss one when digging. One small root will mean a new crop next year. It's a pain!
Crab green or crab grass? Don't kill the crabs! They eat bugs!