Repeating assertions in regular expression

MRAB python at
Tue Jan 3 13:57:39 EST 2012

On 03/01/2012 09:45, Devin Jeanpierre wrote:
>>  \\b\\b and \\b{2} aren't equivalent ?
> This sounds suspiciously like a bug!
>>  Why the wording is "should never" ? Repeating a zero-width assertion is not
>>  forbidden, for instance :
>>>>>  import re
>>>>>  re.compile("\\b\\b\w+\\b\\b")
>>  <_sre.SRE_Pattern object at 0xb7831140>
> I believe this is meant to refer to arbitrary-length repetitions, such
> as r'\b*', not simple concatenations like that. r'\b*' will abort the
> whole match if is run on a boundary, because Python detects a
> repetition of a zero-width match and decides this is an error.
r"\b+" can be optimised to r"\b", but r"\b*" can be optimised to r"". 
r"\b\b", r"\b\b\b", etc, can be optimised to r"\b".

So why doesn't it optimised?

Because every potential optimisation has a cost, which is the time it
would take to look for it.

That cost needs to be balanced against the potential benefit.

How often do you see repeated r"\b"?

Put simply, it doesn't occur often enough to be worth it. The cost
outweighs the potential benefit.

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