I guess the problem is that this is an attribute value
The parses all of that away, I am afraid
The attribute value is abc
the " would be gone after parsing
attribute boundaries can be " or '
if you really need either " or ' and the difference is important, 
I am afraid you would need a regex step before the XSLT (or read the file as a text file in the XSLT)

Met vriendelijke groeten, 
Best regards, 

Geert Bormans

----- Op 14 jan 2021 om 17:24 schreef Martin Honnen martin.honnen@xxxxxx xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx:

On 14.01.2021 17:14, Roger L Costello costello@xxxxxxxxx wrote:

> Here are sample input strings:
>
> ="abc"
> = "abc"
> =       "abc"
>
> That is, an equals, zero or more whitespaces, quote, value, quote
>
> I want everything after the first quote.

My neural network thinks that sounds like

   substring-after($s, '"')

but perhaps I miss something that makes the requirements more complicated.