Lookup base on condition of another table

[#1]

BS_MAP:

LOAD

GL_BS_MAP as GL_BS_MAP

FROM

FERMAT:

LOAD

JOURNAL_ENTRY

IF(LEN(GL_BS_MAP)='5', LOOKUP('LEVEL0', 'GL_BS_MAP', mid(JOURNAL_ENTRY, 9, 5), 'BS_MAP'), LOOKUP('LEVEL0', 'GL_BS_MAP', mid(JOURNAL_ENTRY, 9, 9), 'BS_MAP')) AS LEVEL0

FROM

[\\Fssg51\fsp13\Corporate Treasury Projects\CT-SG\QVD\QLIKMART\FERMAT\L2_FERMAT.QVD]

How can I do this mapping? GL_BS_MAP can be in the format of 11111 or 11111-222. JOURNAL_ENTRY is in the format of 111-222-33333-444. Therefore, if GL_BS_MAP has a length of 5, I want to use LOOKUP('LEVEL0', 'GL_BS_MAP', mid(JOURNAL_ENTRY, 9, 5), 'BS_MAP') for my mapping, but if GL_BS_MAP has a length of 9, I want to use LOOKUP('LEVEL0', 'GL_BS_MAP', mid(JOURNAL_ENTRY, 9, 9), 'BS_MAP'). However, Qlikview prevents me from even checking the len of GL_BS_MAP because it is not in the FERMAT table. How can I go about doing this? Thank you