Re: [FWP] counting in REs

[boyd@Emss.Com (Boyd Nation)]

> A friend of mine recently presented me with three cases to match with
> a regular expression:
> 
>    1)  test for an even number of 'a's in a string
> 
>    2)  test for an odd  number of 'b's in a string
> 
>    3)  test for an even number of 'a's
>        and      an odd  number of 'b's in a string
> 
> Tasks 1 and 2 are pretty easy.  Task 3, however, is rather tricky.  I
> will post some possible approaches in a followup...

Spoiler space:

I haven't seen a followup yet, so I'll throw out this as a starting point
for #3:

/^[^ab]*					# Leading trash
	(a[^ab]*a[^ab]*|			# Discard aa
	b[^ab]*b[^ab]*|				# Discard bb
	a[^ab]*b[^ab]*a[^ab]*b[^ab]*|		# Discard abab
	b[^ab]*a[^ab]*b[^ab]*a[^ab]*|		# Discard baba
	a[^ab]*b[^ab]*b[^ab]*a[^ab]*|		# Discard abba
	b[^ab]*a[^ab]*a[^ab]*b[^ab]*)		# Discard baab
	b[^ab]*$/x				# Find final b

This is obviously extremely brute force.

-- 
Boyd Nation         Energy Management Systems Services     Southern Company
boyd@emss.com                 (205) 257-5789            FAX: (205) 257-3689

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