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From: Jussi Piitulainen <harvesting@is.invalid>
Newsgroups: comp.lang.python
Subject: Re: Newbie: Check first two non-whitespace characters
Date: Fri, 01 Jan 2016 10:16:29 +0200
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otaksoftspamtrap@gmail.com writes:

> I need to check a string over which I have no control for the first 2
> non-white space characters (which should be '[{').
>
> The string would ideally be: '[{...' but could also be something like 
> '  [  {  ....'.
>
> Best to use re and how? Something else?

No comment on whether re is good for your use case but another comment
on how. First, some test data:

  >>> data = '\r\n  {\r\n\t[ "etc" ]}\n\n\n')

Then the actual comment - there's a special regex type, \S, to match a
non-whitespace character, and a method to produce matches on demand:

  >>> black = re.compile(r'\S')
  >>> matches = re.finditer(black, data)

Then the demonstration. This accesses the first, then second, then third
match:

  >>> empty = re.match('', '')
  >>> next(matches, empty).group()
  '{'
  >>> next(matches, empty).group()
  '['
  >>> next(matches, empty).group()
  '"'

The empty match object provides an appropriate .group() when there is no
first or second (and so on) non-whitespace character in the data:

  >>> matches = re.finditer(black, '\r\t\n')
  >>> next(matches, empty).group()
  ''
  >>> next(matches, empty).group()
  ''