Thank you! I wasn’t aware of Instaparse or its use of PEGs [1] which gives you the same sense about parsing ambiguities.

> REXX - I thought this was ingenious specifically for file/text processing

Formally the REXX in Amiga was called ARexx and included extensions [2]. REXX [3] itself is not specifically for file/text processing but enables you to connect different environments [4].

[1] https://en.wikipedia.org/wiki/Parsing_expression_grammar

[2] https://en.wikipedia.org/wiki/ARexx

[3] https://en.wikipedia.org/wiki/Rexx

[4] https://www.ibm.com/docs/en/zos/3.1.0?topic=environments-hos...