How Can I Efficiently Match Strings Starting with '00' but Not '000' in PostgreSQL?

Regular Expressions in PostgreSQL

Regular expressions provide a powerful way to search for patterns in text strings. However, occasionally you may encounter challenges in using them effectively.

One such example is trying to match a string that starts with '00' but where the third character is not '0'. Using the LIKE clause with an expression such as "00[1-9]%" may not work as expected.

To address this, you can employ the regular expression operator (~) and bracket expressions. Consider the following query:

SELECT *
FROM   tbl
WHERE  value ~ '^00[^0]'

Here, ^ indicates that the pattern should match at the start of the string, and ¹ is a character class that matches any character that is not '0'.

Alternatively, for optimal performance, you can use a combination of LIKE and NOT LIKE expressions:

SELECT *
FROM   tbl
WHERE  value LIKE '00%'
AND    value NOT LIKE '000%'

LIKE is generally faster than regular expressions, so it's beneficial to use it when appropriate. Additionally, PostgreSQL can utilize indexes for left-anchored LIKE expressions (e.g., '00%'), which can significantly improve performance for large tables.

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