Some regular expressions for catching typical writing errors – spelling issues (US spelling), repeat words, etc.:
ag --md " a [aeioAEIO]"
ag --md " an [bcdfghjklmnpqrstvxzwyBCDGHJKPQRTVWXYZ]"
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\. '