Re: Use regular expression to retrieve all image tags from a given content

On 4-7-2012 3:03, Tim Chase wrote:

> r = re.compile(r"""
> < # tag opening
> \s* # optional whitespace
> img # the tag
> \b # must end here
> (?: # one of these things:
> \s+ # whitespace
> (?:[a-z][a-z0-9]+:)? # an optional namespace
> src # a "src" attribute
> \s* # optional whitespace
> = # the equals sign
> \s* # optional whitespace
> ( # capture the value
> "[^"]*" # a double-quoted string
> | # or
> '[^']*' # a single-quoted string
> | # or
> [^-a-z0-9._:]*" # per HTML spec
> ) # end of the captured src
> | # or something that's not src
> \s+ # whitespace
> (?:[a-z][a-z0-9]+:)? # an optional namespace
> [a-z0-9]+ # the tag name
> (?: # an optional value
> \s* # optional whitespace
> = # an assignment
> \s* # optional whitespace
> (?: # the value
> "[^"]*" # a double-quoted string
> | # or
> '[^']*' # a single-quoted string
> | # or
> [^-a-z0-9._:]*" # per HTML spec
> ) # end of the captured src
> ) # end of ignored attribute
> )* # zero or more attributes
> \s* # optional whitespace
> (?:/\s*) # an optional self-closing
> > # closing >
> """, re.I | re.VERBOSE)
>
> So, that said, I'm not sure it's much more complex to use a real
> parsing library :-)

Aside from the \b matching positive against ><, this is a syntax
validator and of course when validating syntax you'll use a validating
parser. For all other cases, you'd be more interested in "image tags
with a src attribute", making the use-case quite a bit more simple.

My main beef with modern software is that for the simplest of things one
flees to full-blown libraries which happen to provide some utilities,
but the other 98% of the code from that library is unused. Case in
point, PIL to verify if a file is an image. But my rant alarm went off. :)

>> It's a trade-off you should make a decision on, not just blatantly
>> dismiss regular expressions when a document contains tags or call them
>> complex when they contain more then two characters.
>
> I'm not blithely dismissing them,

But the author I replied to did. I think you and I are on the same page.
Either way, the OP now has some nice examples of how to /refine/ regular
expressions and that's the real craft :).

--
Melvyn Sopacua


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