[R] regex - extracting src url

[Martin Morgan]

On 03/22/2016 12:44 AM, Omar André Gonzáles Díaz wrote:
> Hi,I have a DF with a column with "html", like this:
>
> <IMG SRC="
> https://ad.doubleclick.net/ddm/trackimp/N344006.1960500FACEBOOKAD/B9589414.130145906;dc_trk_aid=303019819;dc_trk_cid=69763238;ord=[timestamp];dc_lat=;dc_rdid=;tag_for_child_directed_treatment=?"
> BORDER="0" HEIGHT="1" WIDTH="1" ALT="Advertisement">
>
>
> I need to get this:
>
>
> https://ad.doubleclick.net/ddm/trackimp/N344006.1960500FACEBOOKAD/B9589414.130145906;dc_trk_aid=303019819;dc_trk_cid=69763238;ord=[timestamp];dc_lat=;dc_rdid=;tag_for_child_directed_treatment=
> ?
>
>
> I've got this so far:
>
>
> https://ad.doubleclick.net/ddm/trackimp/N344006.1960500FACEBOOKAD/B9589414.130145906;dc_trk_aid=303019819;dc_trk_cid=69763238;ord=[timestamp];dc_lat=;dc_rdid=;tag_for_child_directed_treatment=?\"
> BORDER=\"0\" HEIGHT=\"1\" WIDTH=\"1\" ALT=\"Advertisement
>
>
> With this is the code I've used:
>
> carreras_normal$Impression.Tag..image. <-
> gsub("<img.+?src=[\"'](.*?)[\"'].*?>","\\1",carreras_normal$Impression.Tag..image.,
>                                    ignore.case = T)
>
>
>
> *But I still need to use get rid of this part:*
>
>
> https://ad.doubleclick.net/ddm/trackimp/N344006.1960500FACEBOOKAD/B9589414.130145906;dc_trk_aid=303019819;dc_trk_cid=69763238;ord=[timestamp];dc_lat=;dc_rdid=;tag_for_child_directed_treatment=
> ?*\" BORDER=\"0\" HEIGHT=\"1\" WIDTH=\"1\" ALT=\"Advertisement*
>
>
> Thank you for your help.

You're querying an xml string, so use xpath, e.g., via the XML library

 > as.character(xmlParse(y)[["//IMG/@SRC"]])
[1] 
"https://ad.doubleclick.net/ddm/trackimp/N344006.1960500FACEBOOKAD/B9589414.130145906;dc_trk_aid=303019819;dc_trk_cid=69763238;ord=[timestamp];dc_lat=;dc_rdid=;tag_for_child_directed_treatment=?"

`xmlParse()` translates the character string into  an XML document. `[[` 
subsets the document to extract a single element. "//IMG/@SRC" follows 
the xpath specification (this section 
https://www.w3.org/TR/xpath-31/#abbrev of the specification provides a 
quick guide) to find, starting from the 'root' of the document, a node, 
at any depth, labeled IMG containing an attribute labeled SRC.

A variation, if there were several IMG tags to be extracted, would be

   xpathSApply(xmlParse(y), "//IMG/@SRC", as.character)

>
> Omar Gonzáles.
>
> 	[[alternative HTML version deleted]]
>
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