This is an example Reentrancy Guard for teaching purposes

ReentrancyGuard_simpleExample.sol

// SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.7.0 <0.9.0;


contract ReentrancyGuard {
    bool private guardLocked;

    modifier noReentry() {
        require(!guardLocked, "Prevented by noReentry in ReentrancyGuard");
        guardLocked = true;
        _;
        guardLocked = false;
    }
}

Used as an example in This Blog Post