checking a string against multiple patterns

[Gabriel Genellina]

On 18 dic, 09:41, tomasz <tmkm... at googlemail.com> wrote:

> Hi,
>
> here is a piece of pseudo-code (taken from Ruby) that illustrates the
> problem I'd like to solve in Python:
>
> str = 'abc'
> if str =~ /(b)/     # Check if str matches a pattern
>   str = $` + $1    # Perform some action
> elsif str =~ /(a)/ # Check another pattern
>   str = $1 + $'    # Perform some other action
> elsif str =~ /(c)/
>   str = $1
> end
>
> The task is to check a string against a number of different patterns
> (containing groupings).
> For each pattern, different actions need to be taken.
>
> In Python, a single match of this kind can be done as follows:
>
> str = 'abc'
> match = re.search( '(b)' , str )
> if match: str = str[0:m.start()] + m.group(1)    # I'm not sure if
> this way of accessing 'pre-match'
>                                                                # is
> optimal, but let's ignore it now
>
> The problem is that you you can't  extend this example to multiple
> matches with 'elif'
> because the match must be performed separately from the conditional.
>
> This obviously won't work in Python:
>
> if match=re.search( pattern1 , str ):
>   ...
> elif match=re.search( pattern2 , str ):
>   ...
>
> So the only way seems to  be:
>
> match = re.search( pattern1 , str ):
> if match:
>    ....
> else:
>    match = re.search( pattern2 , str ):
>    if match:
>       ....
>    else:
>        match = re.search( pattern3 , str ):
>        if match:
>            ....
>
> and we end up having a very nasty, multiply-nested code.

Define a small function with each test+action, and iterate over them
until a match is found:

def check1(input):
  match = re.search(pattern1, input)
  if match:
    return input[:match.end(1)]

def check2(input):
  match = re.search(pattern2, input)
  if match:
    return ...

def check3(input):
  match = ...
  if match:
    return ...

for check in check1, check2, check3:
  result = check(input)
  if result is not None:
    break
else:
  # no match found

--
Gabriel Genellina