About 10 years ago I had to write a script to reconcile trade entries from various brokers back into our central system.

You are 100% correct on the "100 to 10" ratio on test cases.

PLUS, the ways in which broker files can break due to:

- random carriage returns

- different date formats

- time zones

- etc etc

and regexes become both great and terrifying at the same time.

One pattern I did find useful:

regex + if/then

e.g. if (regex is true) then if (regex2 is true) then