Regex Hell to LLM Function Calling: My Data Extraction Journey

A few months ago, I had a problem that made me question my career choices. I was staring down a folder with 500+ PDF invoices. Each one had the same fields - invoice number, date, line items, totals - but the formatting was different on every single one. Some had tables, some had columns, some were just text blocks with a dash separator. The client wanted all of it in a clean JSON array. At first, I thought: "Regex. I'll write a few patterns, test them, and be done in a day." That was naive....