Selecting features whose text is equal to the text after certain word in QGIS

Question

I have origin-destination lines and I'm looking for intra-city lines or lines whose origin and destination are in the same city.

A sample intra-city feature would be "Quezon_City to Quezon_City" and inter-city would be "Quezon_City to CALOOCAN_CITY".

I'd like some help in writing an expression that selects all features whose string before "to" is the same with the string after it using Python or Expression Builder.

Answer 1

@Taras has already given an answer that solves your problem. Let me give you another one using PyQGIS.

I have an attribute table with 2 fields: descriptions which corresponds to your text and equal to indicate whether the text before to is identical to the text after to.

layer = iface.activeLayer() # your layer
layer.startEditing()

# Loop on each feature of the layer
for feat in layer.getFeatures():
    feat_text = feat["description"] # your text
    pos_to = feat_text.find("to") # find the position of the word "to"
    before_to = feat_text[:pos_to].replace(" ", "") # Extract the text before the previous position and remove blank lines
    after_to = feat_text[pos_to+2:].replace(" ", "") # Extract the text after 
    
    # Update of the "equal" field depending the text before and after
    if before_to == after_to:
        feat["equal"] = 'yes'
        layer.updateFeature(feat)
    else:
        feat["equal"] = 'no'
        layer.updateFeature(feat)
    
layer.commitChanges()

Answer 2

Currently I have two ideas how to tackle your issue

Case 1. Only working with an Attribute table where you will need to use the following expression

if(left("test",regexp_match("test",'\\ to ')-1) = right("test",regexp_match("test",'\\ to ')-1), true, false)

Case 2. Working with geometries of lines and polygons, of course if you have cities as polygons.

So, geometrically check if starting and ending points of the "origin-destination"-lines are both within those polygons. In PostGIS-interpretation it might be something like

st_within(start_point(line.geometry), polys.geometry)
AND st_within(end_point(line.geometry), polys.geometry)