Invoice Field Regex Generator in Python for OCR Text

import re
from datetime import datetime

CUR = re.escape("$")

INVOICE_NO = re.compile(r"(?:Invoice No|Invoice #|Inv No)\s*[:#.]?\s*([A-Z0-9][A-Z0-9\-/]{3,24})", re.I)
PO_NO = re.compile(r"(?:PO Number|Purchase Order|P.O. No)\s*[:#.]?\s*([A-Z0-9][A-Z0-9\-/]{3,24})", re.I)
DATE_TXT = re.compile(r"(?:Invoice Date|Date)\s*[:.]?\s*(\d{4}-\d{2}-\d{2}|\d{1,2}[/.-]\d{1,2}[/.-]\d{2,4})", re.I)
TOTAL = re.compile(r"(?:Total Due|Amount Due|Grand Total|Total)\s*[:.]?\s*" + CUR + r"?\s*([0-9]{1,3}(?:,[0-9]{3})*(?:\.[0-9]{2})|[0-9]+(?:\.[0-9]{2})?)", re.I)
LINE_AMT = re.compile(r"^.*?" + CUR + r"?\s*([0-9]{1,3}(?:,[0-9]{3})*\.[0-9]{2})\s*$", re.M)

def first(pattern, text):
    m = pattern.search(text)
    return m.group(1).strip() if m else None

def parse_date(raw):
    if not raw:
        return None
    for fmt in ("%Y-%m-%d", "%d/%m/%Y", "%d-%m-%Y", "%d.%m.%Y", "%m/%d/%Y", "%d/%m/%y"):
        try:
            return datetime.strptime(raw, fmt).date().isoformat()
        except ValueError:
            continue
    return None

def parse_amount(raw):
    return float(raw.replace(",", "")) if raw else None

def parse_invoice(text):
    result = {
        "invoice_number": first(INVOICE_NO, text),
        "po_number": first(PO_NO, text),
        "invoice_date": parse_date(first(DATE_TXT, text)),
        "total": parse_amount(first(TOTAL, text)),
    }
    result["flags"] = []
    for key in ("invoice_number", "invoice_date", "total"):
        if result[key] is None:
            result["flags"].append("missing_" + key)
    return result

def check_total(text, parsed, tolerance=0.01):
    amounts = [parse_amount(a) for a in LINE_AMT.findall(text)]
    if not amounts or parsed["total"] is None:
        return "cannot_check"
    # the last amount on a page is usually the total, so exclude it from the sum
    body = amounts[:-1]
    if abs(sum(body) - parsed["total"]) <= tolerance:
        return "ok"
    return "review"

if __name__ == "__main__":
    sample = open("invoice.txt", encoding="utf-8").read()
    data = parse_invoice(sample)
    data["total_check"] = check_total(sample, data)
    print(data)