Use split, like so:

>>> filecontent = "13P397;Fotostuff;t;IBM;IBM lalala 123|IBM lalala 1234;28.000 things;;IBMlalala123|IBMlalala1234";
>>> items = filecontent.split(";");
>>> items;
['13P397', 'Fotostuff', 't', 'IBM', 'IBM lalala 123|IBM lalala 1234', '28.000  things', '', 'IBMlalala123|IBMlalala1234']
>>> 

I'm a bit unsure as what you wanted to do in the last step, but perhaps something like this?

>>> [(i, e) for i,e in enumerate(items) if 'IBMlalala123' in e]
[(7, 'IBMlalala123|IBMlalala1234')]
>>> 

Use split, like so:

>>> filecontent = "13P397;Fotostuff;t;IBM;IBM lalala 123|IBM lalala 1234;28.000 things;;IBMlalala123|IBMlalala1234";
>>> items = filecontent.split(";");
>>> items;
['13P397', 'Fotostuff', 't', 'IBM', 'IBM lalala 123|IBM lalala 1234', '28.000  things', '', 'IBMlalala123|IBMlalala1234']
>>> 

I'm a bit unsure as what you wanted to do in the last step, but perhaps something like this?

>>> [(i, e) for i,e in enumerate(items) if 'IBMlalala123' in e]
[(7, 'IBMlalala123|IBMlalala1234')]
>>> 

UPDATE: If I get your requirements right on the second attempt: To find all lines in file having 'IBMlalala123' as any one of the semicolon-separated fields, do the following:

>>> with open('big.file', 'r') as f:
>>>   matching_lines = [line for line in f.readlines() if 'IBMlalala123' in line.split(";")]
>>> 

Use split, like so:

>>> filecontent = "13P397;Fotostuff;t;IBM;IBM lalala 123|IBM lalala 1234;28.000 things;;IBMlalala123|IBMlalala1234";
>>> filecontent.split(";")
['13P397', 'Fotostuff', 't', 'IBM', 'IBM lalala 123|IBM lalala 1234', '28.000  things', '', 'IBMlalala123|IBMlalala1234']
>>> 

Use split, like so:

>>> filecontent = "13P397;Fotostuff;t;IBM;IBM lalala 123|IBM lalala 1234;28.000 things;;IBMlalala123|IBMlalala1234";
>>> items = filecontent.split(";");
>>> items;
['13P397', 'Fotostuff', 't', 'IBM', 'IBM lalala 123|IBM lalala 1234', '28.000  things', '', 'IBMlalala123|IBMlalala1234']
>>> 

I'm a bit unsure as what you wanted to do in the last step, but perhaps something like this?

>>> [(i, e) for i,e in enumerate(items) if 'IBMlalala123' in e]
[(7, 'IBMlalala123|IBMlalala1234')]
>>>