How to 'evaluate' backspaces in python?

Question

I have byteslike 'foo\x20\x20\x08\x08bar'

I need have the backspaces ('\x08') evaluated when and only when they are lead by identical number of spaces ('\x20').

x = re.sub('\x20+\x08+', '', t) is the naive way of doing this, but fails to produce correct output when t = 'foo\x20\x20\x08'

Is there a way to define a regular expression that takes the length of a previous group in to account when matching the second group or do I need do this manually with re.finditer & re.span() and then manually re-checking the preceding blocks?

behzad.nouri · Accepted Answer · 2014-09-25 11:38:14Z

2

An alternative is to pass a lambda to re.sub:

>>> pat ='(\x20+)(\x08+)' 
>>> repl = lambda m: m.group(1)[:-len(m.group(2))]

now:

>>> re.sub(pat, repl, 'foo\x20\x20\x08bar')
'foo bar'
>>> re.sub(pat, repl, 'foo\x20\x20\x08\x08bar')
'foobar'
>>> re.sub(pat, repl, 'foo\x20\x20\x08\x08\x08bar')
'foobar'

answered Sep 25, 2014 at 11:38

behzad.nouri

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2 Comments

behzad.nouri Over a year ago

@vks see re.sub and the 2nd example there. The repl argument can be a function which receives a match object and returns a string.

vks Over a year ago

that i know.I have used it before.the use of lambda is somewhat confusing.Also when exactly the space has to be put?

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