import re
text = '''
 
547 U18101MH1988PTCO50073 =RAINBOW GARMENTS AND EXPORTS (INDIA) PRI
548 UU24230MH2004PTC147574 RAINBOW HERBAL AND RESEARCH PRIVATE LIMI
549 U55200MH1992PTCO067327 RAINBOW HOLIDAY-INNPVT.LTD.
 
550 U15100MH2004PTC148619 RAINBOW HORTICULTURE PRIVATE LIMITED
 
551 US9S99S9MH2000PTC124660 RAINBOW INFOTECH (INDIA) PRIVATE LIMITED
552 U70200MH2011PTC221779 RAINBOW INFRAVENTURES PRIVATE LIMITED
553 U15130MH2004P1C14/013 RAINBOW JELSTAR FOODS PRIVATE LIMITED
'''
 
print( re.findall(r'(?<!\S)(?=\S{19,21}(?!\S))U\S*\d{5}\S*\d(?!\S)',text) )

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