Pre Tokenization Regex

Why does SentencePiece prepend a special space symbol (▁) to tokens, and what would break without it?

Which tokenizer can learn merges that span whitespace boundaries by design?

What happens to emoji ZWJ sequences under cl100k_base vs. o200k_base pre-tokenization?

What is the primary purpose of regex-based pre-tokenization in cl100k_base, and what constraint does it impose on BPE?