How do I pass a complex regexp to Grep for Windows?

Hi -

Regarding the version of grep distributed here, which I am running from the Windows XP command line:
http://pages.interlog.com/~tcharron/grep.html

If I run this command:
grep -S -"size" *.flam3

I get results okay (it prints every file name and line where there is
a match - which in my case is every *.flam3 file in the directory).

But if I run this command -
grep -S "(?<=size=")[0-9]+" *.flam3

I get this error:
E:\Program Files\Grep\grep.exe: : Not enough memory

(My grep install is on an E: drive and included in the windows global
PATH environment variable.)

According to this "PowerGREP" tool and "regexBuddy" software I'm
using, that regular expression -
(?<=size=")[0-9]+

- should match the first number after size=" (so it will match 800) in
the below:
time="0" size="800 592" center="-0.408976 -0.305538"

- and alternately, this expression
(?<=size="[0-9]+ )[0-9]+

- should match the second number (or 592) in the same area:
time="0" size="800 592" center="-0.408976 -0.305538"

What am I doing wrong, or what may be wrong?

Puzzling: I can get a match from this command (it outputs a 1, meaning "match found" -

grep -c "size=""" electricsheep.243.06908.flam3

or from this:

grep -c [0-9] electricsheep.243.06908.flam3

But again, not from that first problem regex mentioned.  I'm suspecting the problem has to do with either how to tell it to look for a quote mark, or how to set off the regex from the quote marks required around the expression.

Also, when I get it narrowed down to finding a match, I'll wonder how to display only the match text (not the whole filename and line that matches), as I'll be passing the whole thing into a FINDSTR function in a Windows batch file, like so:

FOR /F "usebackq tokens=* delims=*" %%X IN (`grep "matchtext" file.flam3`) DO      (
      SET var=%%X
      ECHO var is !var!
                                                                                                            )

Thank you!