[MDEV-24987] Regexp Look-ahead is working without a fixed-length regexp Created: 2021-02-25  Updated: 2021-03-09  Resolved: 2021-03-09

Status: Closed
Project: MariaDB Server
Component/s: Documentation
Affects Version/s: 10.5.5
Fix Version/s: N/A

Type: Bug Priority: Trivial
Reporter: Christian Proust Assignee: Ian Gilfillan
Resolution: Fixed Votes: 0
Labels: documentation


 Description   

In the PCRE page, it is written:

Note that look-ahead and look-behind assertions can only be of fixed length; you cannot have repetition operators or alternations with different lengths

It is true that for a look-behind, the pattern should have a fixed-length. However, for a look-ahead, this is not requested.

More information from PERL documentation.

Example:

WITH log(X) AS (
 
	VALUES
 
	-- 'over' does not match because 'cute' does not end with a digit
 
	('Do you know the droid? It is over-cute <3'),
 
	('I think it is BB-8.'),
 
	-- 'BB' does not match because it is preceded by 'not'.
 
	('No it is not BB-8. It is R2-D2.')
 
)
 
-- Search a word preceding by 'is ' and followed by '-\\w*\\d'
 
SELECT REGEXP_SUBSTR(X, 'is \\K\\w+(?=-\\w*\\d)')
 
FROM log;

Returns:

(''),
 
('BB'),
 
('R2')


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