Some regular expressions for catching typical writing errors (spelling issues, repeat words, etc.):
ag --md " a [aeioAEIOU]"
ag --md -i " an [bcdfghjklmnpqrstvxzwy]"
ag --md -i "\s\b(\w+) +\1\b"
ag --md -i "\s\b(\w+ +\w+) +\1\b"
ag --md '\w+isation\b'
ag --md 'e\.g\. '
ag --md 'i\.e\. '