+ in regular expression

[Evan Driscoll]

On 10/05/2012 04:23 AM, Duncan Booth wrote:
> A regular expression element may be followed by a quantifier.
> Quantifiers are '*', '+', '?', '{n}', '{n,m}' (and lazy quantifiers
> '*?', '+?', '{n,m}?'). There's nothing in the regex language which says
> you can follow an element with two quantifiers.
In fact, *you* did -- the first sentence of that paragraph! :-)

\s is a regex, so you can follow it with a quantifier and get \s{6}. 
That's also a regex, so you should be able to follow it with a quantifier.

I can understand that you can create a grammar that excludes it. I'm 
actually really interested to know if anyone knows whether this was a 
deliberate decision and, if so, what the reason is. (And if not -- 
should it be considered a (low priority) bug?)

Was it because such patterns often reveal a mistake? Because "\s{6}+" 
has other meanings in different regex syntaxes and the designers didn't 
want confusion? Because it was simpler to parse that way? Because the 
"hey you recognize regular expressions by converting it to a finite 
automaton" story is a lie in most real-world regex implementations (in 
part because they're not actually regular expressions) and repeated 
quantifiers cause problems with the parsing techniques that actually get 
used?

Evan