Postgresql - Chain multiple regex_replace functions in single query? - postgresql

Question

Using Postgresql 11.6. I have values in tab_a.sysdescr that I want to convert using regex_replace and update those converted values into tab_b.os_type.
Here is table tab_a that contains the source string in sysdescr :
hostname | sysdescr |
-------------+-----------------+
wifiap01 | foo HiveOS bar |
switch01 | foo JUNOS bar |
router01 | foo IOS XR bar |
Here is table tab_b that is the target for my update, in column os_type :
hostname | mgmt_ip | os_type
-------------+--------------+---------
wifiap01 | 10.20.30.40 |
switch01 | 20.30.40.50 |
router01 | 30.40.50.60 |
This is example desired state for tab_b :
hostname | mgmt_ip | os_type
-------------+--------------+---------
wifiap01 | 10.20.30.40 | hiveos
switch01 | 20.30.40.50 | junos
router01 | 30.40.50.60 | iosxr
I have a working query that will work against a single os_type. In this example, HiveOS :
UPDATE tab_b
SET os_type = (
SELECT REGEXP_REPLACE(sysdescr, '.*HiveOS.*', 'hiveos')
FROM tab_a
WHERE tab_a.hostname = tab_b.hostname
)
WHERE EXISTS (
SELECT sysdescr
FROM tab_a
WHERE tab_a.hostname = tab_b.hostname
);
What I can't figure out is how I can "chain" multiple regex_replace functions together into a single query, or via nested sub-queries. Adding 'OR' after that SELECT REGEX_REPLACE line doesn't work, and haven't been able to find examples online of something like this.
End-goal is a single query function that will replace the strings as specified, updating the replaced string on all rows in tab_b. I was hoping to avoid having to delve into PL/Python but if that is the best way to solve this, that's okay. Ideally, I could define a third table that contains the pattern and replacement_string arguments - and could iterate over that somehow.
Edit: Example of what I am trying to accomplish
This is not valid code, but hopefully demonstrates what I am trying to accomplish. A single query that can be executed once, and will translate/transform every sysdescr in a table into proper values for os_type in a new table.
UPDATE tab_b
SET os_type = (
SELECT REGEXP_REPLACE(sysdescr, '.*HiveOS.*', 'hiveos') OR
SELECT REGEXP_REPLACE(sysdescr, '.*JUNOS.*', 'junos') OR
SELECT REGEXP_REPLACE(sysdescr, '.*IOS XR.*', 'iosxr')
FROM tab_a
WHERE tab_a.hostname = tab_b.hostname
)
WHERE EXISTS (
SELECT sysdescr
FROM tab_a
WHERE tab_a.hostname = tab_b.hostname
);

Answer 1

If foo and bar are consistent in all rows (as indicated in your example), then this should work:
postgres=# SELECT lower(replace(regexp_replace('foo IOS XR bar','foo (.*) bar','\1'),' ',''));
lower
-------
iosxr
(1 row)
In short, this does the following:
Trim off foo and bar from the front and back with regexp_replace()
Remove the spaces with replace()
Lower-case the text with lower()
If you need to do anything further to remove foo and bar, you can nest the string functions as demonstrated above.

Answer 2

I was able to solve this using a third table (lookup table). It contains two columns, one holding the match string and one holding the return string.
New table tab_lookup:
id | match_str | return_str
----+-----------------------------------------------+------------
1 | HiveOS | hiveos
2 | IOS XR | iosxr
3 | JUNOS | junos
5 | armv | opengear
6 | NX-OS | nxos
7 | Adaptive Security Appliance | asa
17 | NetScreen | netscreen
19 | Cisco Internetwork Operating System Software | ios
18 | Cisco IOS Software | ios
20 | ProCurve | hp
21 | AX Series Advanced Traffic Manager | a10
22 | SSG | netscreen
23 | M13, Software Version | m13
24 | WS-C2948 | catos
25 | Application Control Engine Appliance | ace
Using this query I can update tab_b.os_type with the appropriate value from tab_lookup.return_str:
UPDATE tab_b
SET os_type = (
SELECT return_str
FROM tab_lookup
WHERE EXISTS (
SELECT regexp_matches(sysdescr, match_str)
FROM tab_a
WHERE tab_a.hostname = tab_b.hostname
)
);
The only catch I have encountered is that there must be only one match against a given row. But this is easily accomplished by verbose match_str values. E.g, don't use 'IOS' but instead use 'Cisco IOS Software'.
All in all, very happy with this solution since it provides an easy way to update the lookup values, as more device types are added to the network.