Examples

• By first letter: (^[a-z])
• By first two letters: (^[a-z]{2})
• By second letter: ^[a-z]([a-z])
• By first and third letter: (^[a-z])[a-z]([a-z]).*$
• By last letter: ([a-z]$)
• By last three letters: ([a-z]{3}$)
• By first letter and second letter: (^[a-z])([a-z]).*$
• By first letter and last letter: (^[a-z]).*([a-z]$)
• By first letter of words ending "ing": (^[a-z]).*(?:ing$)
• Words starting c-z, followed by ab, followed by any letters except ab: (^[^ab]{3,})ab([^ab]+$)
• Prefixes and suffixes for "total": ([a-z]*)total([a-z]*)
• Words with a three letter sequence repeated three or more times: .*([a-z]{3,}).*\1.*\1.*
• Words containing all five vowels: (?=\w*?a)(?=\w*?e)(?=\w*?i)(?=\w*?o)(?=\w*?u)([a-z]+)
• Words with three or more repeated letters (not necessarily distinct): .*([a-z])\1.*([a-z])\2.*([a-z])\3.*([a-z])\4.*
• Words of four or more letters from [eginpux] that contain "e": (?=.*e)(^[eginpux]{4,}$). See NYT Spelling Bee,