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:

enter image description here

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

• Ivan M: How do I rewrite this expression for the opposite process? From 8F_16B_17B to 8,-16,-17
• Kadir Şahbaz: You'd better use a regular expression for this. I don't have enough experience on regular expressions. You can ask this as another question.

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:

   enter image description here

2. Use it in the Field Calculator:

   enter image description here

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

result

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