🐛 Bug: diff formatter stops at first colon

[bmarker]

Bug Report Checklist

• I have read and agree to Mocha's Code of Conduct and Contributing Guidelines
• I have searched for related issues and issues with the faq label, but none matched my issue.
• I have 'smoke tested' the code to be tested by running it outside the real test suite to get a better sense of whether the problem is in the code under test, my usage of Mocha, or Mocha itself.
• I want to provide a PR to resolve this

Expected

For this assertion:

    expect(1, 'Test: expected something').to.equal(2)

Expected output:

      Test: expected something
      + expected - actual

      -1
      +2

Actual

Actual output:

      Test
      + expected - actual

      -1
      +2

Minimal, Reproducible Example

import { expect } from 'chai'

describe('reporter', function () {
  it.only('should log', function () {
    expect(1, 'Test: expected something').to.equal(2)
  })
})

Versions

chai@5.2.0
mocha@11.7.0

Additional Info

This ticket matches but is closed:
#3531

If you rather re-open that ticket, I'm fine with closing this one as a duplicate.

The regex here appears to have unexpected behavior when the message contains a colon before ": expected".

      var match = message.match(/^([^:]+): expected/);
      msg = '\n      ' + color('error message', match ? match[1] : msg);

No Match

For example:

expect(1, 'Test: something').to.equal(2)

The message is: "Test: something: expected 1 to equal 2". The match is falsy even though it ends with ": expected ..." so the whole message is used:

      AssertionError: Test: something: expected 1 to equal 2
      + expected - actual

      -1
      +2

This looks reasonable until you look at the other use cases. This is actually the output I wanted originally but looking at the code and other cases I don't think this is the expected result.

Unexpected Match

If the custom message includes ": expected", you get something really strange:

    expect(1, 'Test: expected something').to.equal(2)

The message is: "'Test: expected something: expected 1 to equal 2" and the capture group is "Test" so you just get:

      Test
      + expected - actual

      -1
      +2

Expected Match

The "normal" case uses just the custom message:

    expect(1, 'Test').to.equal(2)

The message is "Test: expected 1 to equal 2" and the capture group is "Test" so you get just the custom message:

      Test
      + expected - actual

      -1
      +2

Suggestion

Assuming the expected behavior is to extract the custom message and drop the ": expected ..." at the end, like the example with no colon, this regex seems to work better:

      var match = message.match(/^(.+): expected/);

[JoshuaKGoldberg]

Thanks for the report and suggested new (& simpler) regexp! I can see why you'd prefer the suggested behavior. But, makes the message with a colon(:) case from #3531 less informative.

https://github.com/JoshuaKGoldberg/repros/tree/mocha-with-different-diff-colon-formatting

  2) Should print message with a colon(:):

-     Error: message with a colon(:): expected something else
+     message with a colon(:)
      + expected - actual

  3) reporter
       should log with colon:

-     Test
+     Test: expected something
      + expected - actual

#3531 has been closed for almost 7 years without activity, and this is the first request for a behavior change since then. I'm thinking changing the current behavior probably isn't worth the risk of breakage. We're trying not to change too much with Mocha (#5027). Mocha has so many dependents from so many years of usage that practically every little quirk is depended upon somewhere.

In other words: if we were designing a new test runner from scratch I would almost certainly go for the suggested change. But since Mocha is so established, I don't think we can make this change.

A rueful -1 from me, with request for more opinions from @mochajs/committers.