![]() Why is each returned row an array, rather than a string? Because if we use groups to capture parts of the text, the array will contain the groups: CREATE TABLE Stuff (id SERIAL, thing TEXT) ![]() This indicates that there were three matches. Notice how regexp_matches, because of the g option, returned three rows, with each row containing a single (one-character) array. SELECT id, thing FROM Stuff WHERE thing ~* '^ This operator, as mentioned above, is often used to include or exclude rows in a query’s WHERE clause: CREATE TABLE Stuff (id SERIAL, thing TEXT) ![]() Select 'abcq' ~* '^]+$' - returns "false" In addition to the standard character classes, we can also use POSIX-style character classes: select 'abc' ~* '^]$' - returns "false" Thus, you can say: select 'abc' ~ 'a.c' - returns "true" All of these operators return true or false: In each case, the text string to be matched should be on the left, and the regexp should be on the right. PostgreSQL comes with four regexp operators. By contrast, the regexp functions are meant to retrieve some or all of a string from a column’s text value. PostgreSQL’s operators are generally aimed at finding whether a particular regexp matches text, in order to include or exclude result rows from an SQL query. Where things are truly different in PostgreSQL’s implementation is the set of operators and functions used to work with regexps. (This was likely done to avoid conflicts with the ASCII backspace character.) However, there are differences between the regexp syntax -for example, PostgreSQL’s word-boundary metacharacter is \y whereas in Python and Ruby, it is \b. Thus, + is a metacharacter in PostgreSQL, whereas \+ is a plain “plus” character. PostgreSQL’s regexp syntax is similar to that of Python and Ruby, in that you use backslashes to neutralize metacharacters. If there is a match, PostgreSQL returns “true.” Thus, you will create a string (using single quotes only you should never use double quotes in PostgreSQL), and then match that to another string. Regexps in PostgreSQL are defined using strings. ![]() Regexps in PostgreSQL are a hidden gem, one which many people don’t even know exists, but which can be extremely useful. It can be used to test which rows match certain criteria, but it can also be used to retrieve selected text from columns inside of a table. That said, PostgreSQL includes a powerful regexp engine. PostgreSQL isn’t a language per se, but rather a relational database system. I’ve used these many times over the years, and it’s quite possible that you’ll also find them to be of assistance when writing queries. I’m thus presenting an excerpt from the book, providing an overview of PostgreSQL’s regexp operators and functions. And even more are surprised by the variety of ways in which they can use regexps from within PostgreSQL. Others, once they take a look, are surprised by how powerful the engine is. When I tell people this, they often say, “PostgreSQL? Really?!?” Many are surprised to hear that PostgreSQL supports regexps at all. The book (similar to my earlier ebook, “ Practice Makes Python“) contains 50 exercises to improve your fluency with regular expressions (“regexps”), with solutions in Python, Ruby, JavaScript, and PostgreSQL. Introduction to machine learning in PythonĪfter months of writing, editing, and procrastinating, my new ebook, “Prac tice Makes Regexp” is almost ready.Introduction to Python (for experienced coders).
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