Noel Jones put forth on 3/7/2011 9:49 AM:
> On 3/7/2011 8:13 AM, Stan Hoeppner wrote:
>> Noel Jones put forth on 3/7/2011 7:00 AM:
>>> On 3/7/2011 4:47 AM, Stan Hoeppner wrote:
>>>>
>>>> I was taught to always start my expressions with "/^" and end them with
>>>> "$/". Why did Steven teach me to do this if it's not necessary?
>>>
>>> That's good advice when you're actually matching something.
>>
>> Ok, so if I'm doing what I've heard called a "fully qualified regular
>> expression", WRT FQrDNS matching, should I use the anchors or not?
>> postmap -q says these all work (the actuals with action and text that
>> is).
>>
>> /^(\d{1,3}-){3}\d{1,3}\.dynamic\.chello\.sk$/
>> /^(\d{1,3}\.){4}dsl\.dyn\.forthnet\.gr$/
>> /^(\d{1,3}-){4}adsl-dyn\.4u\.com\.gh$/
>> /^[\d\w]{8}\.[\w]{2}-[\d]-[\d\w]{2}\.dynamic\.ziggo\.nl$/
>> /^(\d{1,3}\.){4}dynamic\.snap\.net\.nz$/
>> /^pppoe-dyn(-\d{1,3}){4}\.kosnet\.ru$/
>
> In these examples, you're explicitly matching something at the start
> and/or end of the string. Using the anchors is correct and recommended.
>
>
>>
>>> The special case of .* means, as you know, "anything or nothing".
>>> There's never a case where it's necessary to explicitly match a leading
>>> or trailing "anything or nothing".
>>
>> What of the case where you want to match something in the middle of the
>> input string, with extra junk on both ends?
>
> If you're looking for a string that contains foo anywhere, simply
> /foo/
> with no anchors.
>
>
>>
>>> Consider:
>>> /^.*foo$/
>>> match the string beginning with anything or nothing, ending with foo.
>>>
>>> can always be simplified to:
>>> /foo$/
>>> match the string ending with foo.
>>>
>>> This works the same without the ending $ anchor (contains foo, rather
>>> than ends with foo), but helps the illustration.
>>
>> So, in my examples above, given we're matching rDNS patterns, are the
>> anchors necessary, or helpful? If not using them means "contains", then
>> they should still match. What advantage is there to using the anchors
>> when matching rDNS patterns? Any?
>
> You use anchors to reduce the chance of a false positive. A side
> benefit is improved performance.
>
> Any pattern that matches with the anchors will still match without the
> anchors, but may match additional input that you don't intend to match.
> In the case of the rDNS patterns, a FP is unlikely (but possible, more
> so with the shorter patterns).
>
> In other cases, such as matching a sort bare domain name, a FP may be
> very likely without anchors.
>
> "best practice" is to use the anchors when you can, ie. what you're
> matching will always be at the beginning and/or end of the input
> string. Never use ^.* or .*$.
Excellent explanations. Thank you Noel.
--
Stan
