Michael Hutchinson wrote:
> > body NICE_GIRL_01 /Hello! I am (?:bored|tired) (?:today|this
> > (?:afternoon|evening)|tonight)\./

>
> Forgive my ignorance, but what does the question mark and colon do at
> the start of the brackets? I have (bored|tired) in my own rules, so how
> does (?:bored|tired) affect the outcome?


Using (?: avoids creating backreferences. It should be slightly
faster if the backreference is not used.

(?:bored|tired)

Is the same as:

(bored|tired)

But without creating \1 or $1 reference to it.

SpamAssassin is written in Perl and uses PCRE (Perl Compatible Regular
Expressions). Those are not quite the same as standard Extended
Regular Expressions. For a full description see the 'perlre' man page.

man perlre

"(?attern)"
"(?imsx-imsxattern)"
This is for clustering, not capturing; it groups
subexpressions like "()", but doesn’t make
backreferences as "()" does. So

@fields = split(/\b(?:a|b|c)\b/)

is like

@fields = split(/\b(a|b|c)\b/)

but doesn’t spit out extra fields. It’s also cheaper
not to capture characters if you don’t need to.

Any letters between "?" and ":" act as flags
modifiers as with "(?imsx-imsx)". For example,

/(?s-i:more.*than).*million/i

is equivalent to the more verbose

/(??s-i)more.*than).*million/i

HTH,
Bob