/^(?: (?:re|fw):\s*(?…

Regular Expression

/^(?:
        (?:re|fw):\s*(?:\w+\s+)?\Q$to\E$
        |(?-i:\Q$to\E)\s*[,:;!?-](?:$|\s)
        |\Q$to\E$
        |,\s*\Q$to\E[,:;!?-]$
  )/ix;
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