Following the discussion here I propose to add 3 new string methods: str.trim, str.ltrim, str.rtrim
Another option would be to change API for str.split method to work correctly with sequences.

In [1]: def ltrim(s, seq):

   ...:     return s[len(seq):] if s.startswith(seq) else s
   ...:

In [2]: def rtrim(s, seq):
   ...:     return s[:-len(seq)] if s.endswith(seq) else s
   ...:

In [3]: def trim(s, seq):
   ...:     return ltrim(rtrim(s, seq), seq)
   ...:

In [4]: s = 'mailto:maria@gmail.com'

In [5]: ltrim(s, 'mailto:')
Out[5]: 'maria@gmail.com'

In [6]: rtrim(s, 'com')
Out[6]: 'mailto:maria@gmail.'

In [7]: trim(s, 'm')
Out[7]: 'ailto:maria@gmail.co'

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