Dissolve with expression in QGIS

Question

I have a shapefile with polygons containing UK postcode units, example SL1 1AA. I want to dissolve these into postcode sectors,example SL1 1.

I can use this expression in the filter to get all those postcode units in one sector and dissolve that as the image above.

LIKE 'SL1 1%%'

What i want to do is apply a similar expression with dissolve altering the expression to a regex so it dissolves into SL1 1, SL1 2 etc.

I have an idea how i could tackle this in postgis but wanted to see if i could do this with the QGIS

Answer 1

if you want this to work anywhere in the UK, there are several formats

the following regex :-

[A-Z]{1,2}\d+[A-Z]?\s\d

should match the 6 possible patterns

• EC1A 1BB
• W1A 0AX
• M1 1AE
• B33 8TH
• CR2 6XH
• DN55 1PT

The following works, assuming your field is called pcd.

You can use the regexp_substr() function in QGIS expressions, but this won't work unless

• there is an outer group (i.e. brackets around the whole expression)
• backslashes are doubled (to escape them)

You can use this expression to extract the bit you want

regexp_substr("pcd",'([A-Z]{1,2}\\d+[A-Z]?\\s\\d)') 

I found a few which didn't match, I'm not sure where I got the data from (either OS or ONS; the latter includes lots of expired postcodes which may explain some discrepancies)

A simpler way (and probably faster too, with large datasets) is to simply find the space and take everything up to and including the character after it

 left("pcd" ,strpos("pcd",' ')+1)

Answer 2

Open field calculator and make a new field with the expression left(postcode, 5) Then use the dissolve tool. In the tools' form, uncheck "dissolve all" and use the new field to dissolve by.