[whatwg] RE: pattern attribute

Wrigley, Ave Ave.Wrigley at itn.co.uk
Thu Jul 22 09:25:51 PDT 2004

> Wrigley, Ave writes:
> >> * Whole-pattern matches appears to be significantly more common.
> >> * Forms-design applications that support patterns usually make the 
> >> pattern a whole-pattern rather than substring match.
> > I take the point - but I am not sure I understand why the case for 
> > whole pattern over substring matched differs between when 
> they appear 
> > in a form and when they appear in a script (where substring 
> matching 
> > is more common). Is this just an historical artifact, or is there a 
> > more fundamental reason?
> I'm not totally sure of what you're saying, but I think the 
> answer is that 
> the two problem areas ('pattern matching in script' and 'form 
> validation') 
> do have different requirements. 
> In the first case, you're matching a pattern to a string. It 
> *might* be to 
> validate a form control, or it might not (you might be using 
> a regexp to 
> remove whitespace, or something similar). 
> The way that regexps are designed is (to some extent) 
> counter-intuitive - in 
> the sense that the pattern '1234' can match a string of any 
> length. However, 
> this is well understood, and any semi-experienced programmer will 
> automatically add '^' and '$' assertions into their pattern 
> as required. 
> However, in the 'form validation' case, you're providing a 
> pattern that the 
> form control must match, completely, in order to be valid. 
> There's a much 
> more common use case involving validation of the entire 
> control, compared to 
> validating that part of the control matches a given pattern 
> (a substring 
> match). 
> Regarding the historical angle, there's certainly an argument 
> that today's 
> HTML coders might have more familiarity with ECMAScript 
> pattern matching 
> than with other 'forms designer' tools. However, the two operations - 
> 'writing validation script' and 'writing HTML' - exist (in my 
> mind, at 
> least) in different contexts, and so I don't think that using 
> a different 
> method (substring vs. whole-pattern matching) for the two is 
> particularly 
> confusing, especially when (with the current design) mistakes are 
> highlighted immediately. 
> Consistency is a good argument, but in this case, I think 
> specialising the 
> design to cover the problem area (and therefore introducing a lack of 
> consistency) makes for an easier-to-use (and less prone to 
> failure) design. 
> I hope that's understandable - it's all a bit woolly, I'm afraid! 

Yes - entirely, and I think I broadly agree with you. I suppose my
discomfort is that the value of the pattern attribute is essentially
defined as "a regex, but not quite ...". This is probably because I have
a strong preconception of what a regex is, and a feeling that there are
too many variants on regexes already (not to mention what is happening
with pattern matching in Perl 6)! Still, I think your use case example
probably overcomes this.

Incidentally - do you have any views on regex modifiers?

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