testscript: cmpregex

[myitcv]

Raising as an idea for discussion, because I plan to implement this locally first of all.

Consider:

go test 
env time='\d(\.\d+)?'
cmpregex stdout out.golden

-- out.golden --
$$ go test
Hello, world... from the test!
PASS
ok         _/home/gopher   ${time}s

Would succeed when matched against:

$ go test
Hello, world... from the test!
PASS
ok         _/home/gopher   0.042s

cmpregex is similar to cmpenv, except that it treats $ variable values as a regular expression (with the exception of $$ which expands to $).

Comparison of the arguments to cmpregex then proceeds as follows:

• the reference input has $$ expanded to $
• the reference input is split around $ variables to give the parts of the input
• verify that each $ actually expands to a regular expression that compiles
• walking through the input parts, the reference part is matched against the prefix of the comparison
• if the prefix does not match, a diff is printed for the prefix
• else match the next $-specified regular expression or fail
• continue until all the parts and $-specified regular expressions have been visited and/or the comparison has been consumed

This may or may not belong in the standard library of functions that come with testscript.