pandas DataFrame filter regex

Per the docs,

Arguments are mutually exclusive, but this is not checked for

So, it appears, the first optional argument, items=[0] trumps the third optional argument, regex=r'(Hel|Just)'.

In [194]: df.filter([0], regex=r'(Hel|Just)', axis=0)
Out[194]: 
       0      1
0  Hello  World

is equivalent to

In [201]: df.filter([0], axis=0)
Out[201]: 
       0      1
0  Hello  World

which is merely selecting the row(s) with index values in [0] along the 0-axis.

To get the desired result, you could use str.contains to create a boolean mask, and use df.loc to select rows:

In [210]: df.loc[df.iloc[:,0].str.contains(r'(Hel|Just)')]
Out[210]: 
       0       1
0  Hello   World
1   Just  Wanted

This should work:

df[df[0].str.contains('(Hel|Just)', regex=True)]

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