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How to use regular expression group quantifiers in PostgreSQL

By Selvakumar Arumugam
January 27, 2022

I recently encountered a situation where it was necessary to extract address content from text in HL7 V2 format from a PostgreSQL table’s column. The following example is representative:

||121212^^^2^ID 1|676767||SELVA^KUMAR^^^^|19480203|M||B||123456 SAMPLE ROAD^^New York City^NY^12345^USA^H^^New York||123456-7890|||M|NON|4000|

In order to manipulate our example, the address section needs to be extracted from the HL7 V2 message PID segment for patient demographic information. The segments have delimiters for fields (|), components (^), subcomponents (&) and repetition (~).

Our example has only fields and components delimited by pipe (|) and caret (^). The address contains nine components delimited by ^.

I hoped to do this by applying a regular expression (regex) because the address is in a standard format that regex can match with alphanumeric and caret repetition.

Here is my journey figuring out how to match the data I wanted.

Regex pattern in grep

As a test, I got this regex working with the grep command, which successfully extracts the address section from the content:

$ content='||121212^^^2^ID 1|676767||SELVA^KUMAR^^^^|19480203|M||B||123456 SAMPLE ROAD^^New York City^NY^12345^USA^H^^New York||123456-7890|||M|NON|4000|'
$ echo "$content" | grep -Eo "([A-Za-z0-9 #'.,/-]*\^){8}[A-Za-z0-9 ]*"
123456 SAMPLE ROAD^^New York City^NY^12345^USA^H^^New York

PostgreSQL regex attempt

The PostgreSQL regexp_matches function supports extraction by pattern-matching data from the content. But when I used the same regex pattern with the regexp_matches function, instead of all eight groups, only the eighth value was returned:

=# SELECT regexp_matches('||121212^^^2^ID 1|676767||SELVA^KUMAR^^^^|19480203|M||B||123456 SAMPLE ROAD^^New York City^NY^12345^USA^H^^New York||123456-7890|||M|NON|4000|', '([A-Za-z0-9 #''.,/-]*\^){8}[A-Za-z0-9 ]*', 'g');
 regexp_matches 
----------------
 {^}
(1 row)

More generally, our query is returning the Nth matching group instead of returning all matching groups until the Nth regex group. So if we try to fetch the text matching with 3 groups, the quantifier will return the third field of all match sections instead of the third group itself.

=# SELECT regexp_matches('||121212^^^2^ID 1|676767||SELVA^KUMAR^^^^|19480203|M||B||123456 SAMPLE ROAD^^New York City^NY^12345^USA^H^^New York||123456-7890|||M|NON|4000|', '([A-Za-z0-9 #''.,/-]*\^){3}[A-Za-z0-9 ]*', 'g');
   regexp_matches   
--------------------
 {^}
 {^}
 {"New York City^"}
 {USA^}
(4 rows)

PL/Perl function

Since that doesn’t satisfy our requirements, I tried using a Perl regex through my own PL/Perl function, and got the expected answer:

=# CREATE OR REPLACE FUNCTION perl_regexp_matches (IN str text, IN pattern text) RETURNS text AS $$
    my ($input, $pattern) = @_;
    $output = [$input =~ m/($pattern)/];
    return $output->[0]
$$ LANGUAGE plperl;

=# SELECT perl_regexp_matches('||121212^^^2^ID 1|676767||SELVA^KUMAR^^^^|19480203|M||B||123456 SAMPLE ROAD^^New York City^NY^12345^USA^H^^New York||123456-7890|||M|NON|4000|', '([A-Za-z0-9 #''.,/-]*\^){8}[A-Za-z0-9 ]*');
                    perl_regexp_matches
------------------------------------------------------------
 123456 SAMPLE ROAD^^New York City^NY^12345^USA^H^^New York
(1 row)

But I researched further for a simple solution to achieve the result without using a custom function and the PL/Perl extension.

PostgreSQL regex solution

In Postgres regex syntax, parentheses () create a numbered capture group which leads to returning the contained matching results.

To get the entire matching data, the regex should have a question mark and a colon (?:) added at the beginning of the regex pattern to create a non-capturing group. Then because no group is capturing, instead the complete match is returned:

=> SELECT regexp_matches('||121212^^^2^ID 1|676767||SELVA^KUMAR^^^^|19480203|M||B||123456 SAMPLE ROAD^^New York City^NY^12345^USA^H^^New York||123456-7890|||M|NON|4000|', '(?:[A-Za-z0-9 #''.,/-]*\^){8}[A-Za-z0-9 ]*');
                         regexp_matches                         
----------------------------------------------------------------
 {"123456 SAMPLE ROAD^^New York City^NY^12345^USA^H^^New York"}
(1 row)

That turns out to be what was happening with my PL/Perl function where m/($pattern)/ captures the entire match, and what grep was doing because of its option -o or --only-matching, which prints the matching part of the lines rather than its default of printing the entire line.

And we can also use the Postgres function substring to return the bare text itself instead of arrays as regexp_matches does:

=# SELECT substring('||121212^^^2^ID 1|676767||SELVA^KUMAR^^^^|19480203|M||B||123456 SAMPLE ROAD^^New York City^NY^12345^USA^H^^New York||123456-7890|||M|NON|4000|' FROM '(?:[A-Za-z0-9 #''.,/-]*\^){8}[A-Za-z0-9 ]*');
                         substring
------------------------------------------------------------
 123456 SAMPLE ROAD^^New York City^NY^12345^USA^H^^New York
(1 row)

Reference

• HL7 International website
• PostgreSQL POSIX regular expressions documentation

postgres sql regex hl7 casepointer