Expression to replace characters in QGIS Attribute table

Question

I have a string field. Which requests to put the letter F after the number to all positive digits, and the letter B after minus digits and to delete the sign -.

Finally, I want a result like this:

Answer 1

Use this expression:

array_to_string(
    array_foreach(
        string_to_array( LineCodes, ',' ),
        if(
            left( @element, 1 ) = '-',
            substr( @element, 2 ) + 'B',
            @element + 'F'
            )
        )
    )

string_to_array : splits string into an array using comma
array_foreach : runs given expression for every element in array
array_to_string : concatenates elements into a string separated by comma

Answer 2

You can define your own custom expression function in Python and then use it in the Field Calculator.

@qgsfunction(args='auto', group='Custom', usesGeometry=False)
def custom(input, feature, parent):
    if input is None:
        return None
    numbers = [int(num) for num in input.split(',')]
    def f(num):
        if num > 0:
            return '{}F'.format(num)
        return '{}B'.format(abs(num))
    result = map(f, numbers)
    return ','.join(result)

1. Define it by running the code above as a script in the Python Console:

   

2. Use it in the Field Calculator:

   

Answer 3

You may also try the following expression:

array_to_string(
    array_foreach(
        string_to_array("test"),
        if(
            to_int(@element)>0,
            @element||'F',
            abs(@element)||'B'
            )
        )
    )

See, for example below

P.S. IMHO using the regexp_replace() is pointless because more changes have to be applied rather than substitutions.