Doh! It also plainly says that in the very first sentence--I can't blame the docs for missing that! :)
You're ignoring the first rule of matching: when there is more than one possible match, the match starting earliest in the string is chosen.
How about this revision for 9.6.3.5? I tried to simplify without leaving out any key information or changing the meaning, but I might have gone overboard. Thanks for your help!
Ken
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9.6.3.5. Regular Expression Matching Rules
If there is more than one way an RE can match a string, the first (leftmost) match will be used. If the same starting position yields multiple matches, the match can be either greedy (longest substring) or non-greedy (shortest substring).
Both the RE as a whole and each subexpression can have its own greediness. Greediness is inherited from left to right, so that outer subexpressions take precedence over their component subexpressions. The greediness of an RE is set by the first subexpression that is either greedy or non-greedy.
Parentheses in and of themselves do not effect greediness (although they may affect the precedence of subexpressions).
Most atoms, all constraints, and the quantifiers {m} and {m}? are neither greedy nor non-greedy (because their match lengths are static).
Other normal quantifiers are greedy unless the non-greedy form is explicitly designated. The | operator is greedy. (Use '{m,m}' to force greediness, or '{m,m}?' to force non-greediness).
Lengths are counted in the result string, not the pattern used for matching. An empty string is considered longer than no match at all.
If case-independent matching is specified, the effect is to replace all case-sensitive characters with bracketed expressions containing both cases, or, if already in brackets, the case counterpart is added to the list. (e.g., 'A' acts as '[aA]', '[AB12]' as '[AaBb12]'.)
If newline-sensitive matching is specified, . and bracket expressions using ^ will not match the newline character (so that matches only cross newlines by explicitly using a newline escape) and ^ and $ will match the empty string after and before a newline respectively, in addition to matching at beginning and end of string respectively. But the ARE escapes \A and \Z continue to match beginning or end of string /only/.
Specifying partial newline-sensitive matching affects . and bracket expressions, but not ^ and $. Inverse partial newline-sensitive ("weird") matching affects ^ and $, but not . and bracket expressions. This isn't very useful but is provided for symmetry.
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Tom Lane wrote:
Kenneth Tanzer <[EMAIL PROTECTED]> writes:
But what about these two queries:
SELECT substring('a' FROM 'a?|a?');
This returns a greedy 'a', similar to the example above. But then why does
SELECT substring('ba' FROM 'a?|a?');
return a non-greedy empty string?
You're ignoring the first rule of matching: when there is more than one possible match, the match starting earliest in the string is chosen. The longer-or-shorter business only applies when there are multiple legal ways to form a match starting at the same place. In this case 'a?' can form a legal match at the beginning of the string (ie, match to no characters) and so the fact that a longer match is available later in the string doesn't enter into it.
With regard to the documentation, after re-reading it many times I'd have to say the information is all there, but it's hard to absorb.
I'd agree. This section was taken nearly verbatim from Henry Spencer's man page for the regexp package, and with all due respect to Henry, it's definitely written in geek reference-page-speak. Maybe a few examples would help.
On the other hand, I don't want to try to turn the section into a regexp tutorial --- there are entire books written about regexps (I quite like the O'Reilly one, btw). So there's a bulk-vs-friendliness tradeoff to be made.
I think the main problem is that the term "preference" is used to
discuss greedy/non-greediness, as well as the words greedy &
non-greedy.
Good point. It would help to use only one term.
As an example, here's a couple of different possibilities for the second sentence of the section:
I like this one:
b) If the RE could match more than one substring starting at that point, the match can be either greedy (matching the longest substring) or non-greedy (matching the shortest substring). Whether an RE is greedy or not is determined by the following rules...
Given that intro, there's no need to use the word "preference" at all. Or almost --- what term will you use for "RE with no preference"? Perhaps you can avoid the question by pointing out that greediness only matters for quantifiers, since unquantified REs can only match fixed-length strings.
The point you make here:
c) Like individual components of an RE, the entire RE can be either greedy (matching the longest substring) or non-greedy (matching the shortest substring).
is also important, but probably needs to be a completely separate paragraph containing its own example.
Do you think an edit along these lines would be helpful? If so, I'd be willing to take a shot at re-writing that section. Let me know. Thanks.
Fire away. Please send whatever you come up with to the pgsql-docs list.
regards, tom lane
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