```ts
export function parseSemver(version: string): { major: number; minor: number; patch: number; prerelease: string | null; build: string | null } {
  const match = version.match(/^(\d+)\.(\d+)\.(\d+)(-([a-zA-Z0-9.-]+))?(\+([a-zA-Z0-9.-]+))?$/);
  
  if (!match) {
    throw new Error(`Invalid syntax: ${version}`);
  }

  const [_, major, minor, patch, prerelease = null, build = null] = match;

  return { major: parseInt(major, 10), minor: parseInt(minor, 10), patch: parseInt(patch, 10), prerelease, build };
}

// Example usage:
// console.log(parseSemver("4.6.2")); // { major: 4, minor: 6, patch: 2, prerelease: null, build: null }
// console.log(parseSemver("3.7.5-alpha")); // { major: 3, minor: 7, patch: 5, prerelease: "alpha", build: null }
// console.log(parseSemver("2.0+build123")); // { major: 2, minor: 0, patch: 0, prerelease: null, build: "build123" }
```