I do not know if there is any limit, but, in terms of size of the match, the following Perl one-liner has a regex that happily matches the full text (4.17 MB) of a French translation of the Bible:

$ cat Bible.txt |  perl -e 'local $/; $c = <>; $d = $1 if $c =~/(Gen.+
+tous!)/s; print  length $d;'
4167072

$ wc  Bible.txt
  32324  714470 4167075 Bible.txt
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