Question

In Oracle Regex function REGEXP_SUBSTR , how do I get a Repetition operator to be recognized?

Asked by: Bob_Schmidt_Data_Arch

I am executing the following:
SELECT REGEXP_SUBSTR('2009-452XAB','([[:alpha:]*])') FROM  DUAL
My expected resut is XAB
I have tried moving the splat (*) around without any success.  I have tried using the other parameters of the function as well.

SELECT REGEXP_SUBSTR('2009-452XAB','([[:alpha:]*])') FROM  DUAL

                                  
1:

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Asked On
2009-10-22 at 06:59:55ID24834501
Tags

regex oracle sql function REGEXP_SUBSTR

Topics

Oracle 11.x Database

,

SQL Query Syntax

,

Regular Expressions

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Answers

 

by: mrjoltcolaPosted on 2009-10-22 at 07:11:17ID: 25634393

SELECT REGEXP_SUBSTR('2009-452XAB','[a-zA-Z]+') FROM  DUAL;

 

by: arturrrroPosted on 2009-10-22 at 07:15:12ID: 25634442

or
SELECT REGEXP_SUBSTR('2009-452XAB','[[:alpha:]]+') FROM  DUAL;

 

by: mrjoltcolaPosted on 2009-10-22 at 07:15:49ID: 25634454

Actually now that I see you are using a character class, here is the proper way.

SELECT REGEXP_SUBSTR('2009-452XAB','[[:alpha:]]+') FROM  DUAL;

Your problem was 2-fold

1) * means 0 or more and is non-greedy
2) The * was in the wrong place anyway, it should be outside of the [[]]

 

by: Bob_Schmidt_Data_ArchPosted on 2009-10-22 at 07:27:15ID: 25634592

Ah ha.  Thank you very much.  

Help me out a little further.  I do not understand why a 0-n operator (*) would not return a value when a + 1-n operator (+) does.

And a general question ... I find regex to be totally frustrating.  On the one hand it seems to solve an entire class of problems.  On the other hand, it seems to be very flakey - I have used it in POSTGRES and now in ORACLE and both implementations seem difficult to master.  Is REGEX really reliable or are the rules so complicated to effect in the underlying implementation that it will remain forever somewhat unreliable?

 

by: Bob_Schmidt_Data_ArchPosted on 2009-10-22 at 07:28:07ID: 31644511

I entered a comment in the thread.

 

by: mrjoltcolaPosted on 2009-10-22 at 07:36:32ID: 25634718

>>I do not understand why a 0-n operator (*) would not return a value when a + 1-n operator (+) does.

This is part of the learning curve of regexes, and most moderately seasoned programmers still don't understand it.

Regexes are greedy, but they also will try to match the whole pattern as soon as possible. So when we say greedy, we mean that once a rule is able to be matched, the engine will consume as much of the input text that matches the current state, then move on to the next state, if possible. If not, it will backtrack. So if you look at your example, 0-n legally can be satisfied with the empty string. That is the first state in your rule, and the only state.

Perhaps an example is easier, try both of these:

SELECT REGEXP_SUBSTR('123XYZ','[[:alpha:]]*') FROM  DUAL;
SELECT REGEXP_SUBSTR('ABC123XYZ','[[:alpha:]]*') FROM  DUAL;

In the first one, where do you think the [[:alpha:]]* state was satisfied?


>>On the other hand, it seems to be very flakey - I have used it in POSTGRES and now in ORACLE and both implementations seem difficult to master.  Is REGEX really reliable or are the rules so complicated to effect in the underlying implementation that it will remain forever somewhat unreliable?


Regex engines are Finite State Automata. This, by definition, is the opposite of flaky.

Now, as to different implementations, yes, some regex engines have quirks, but most use the Perl5 "flavor" nowadays so if you learn that, as well as the POSIX character classes (the ones surrounded by colons, like :alpha:) then you can take that anywhere you go.

 

by: mrjoltcolaPosted on 2009-10-22 at 07:38:54ID: 25634749

AND once you master regexes, your power as a programmer and DBA are multiplied over those who do not know regexes. You are exactly right, there are a whole class of problems that are simply trivial to solve with a regex, that take many lines of code and/or iteration or other constructs to solve.

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