Ok, this is quite interesting. I've tried all of the suggested approaches, and based on a sample of actual text that I need to run on, the following is the overall best performing: 
$str =~ /\A.{0,995}?<html/i

[download]
(I added the '<' to the text to make it more specific, and also removed the /s qualifier -- in my situation, both of these tweaks boosted performance.)

But now the plot thickens ... what I just realized is that sometimes my content is gzipped (i.e. served by an Apache web server with mod_deflate, aka "Content-Encoding: gzip", or deflate, or compress). I have used some good CPAN modules that inflate this type of content, but now I'm faced with the same dilemna ... if I've got a 20KB gzipped file (100KB inflated), and I still only care about checking the first 1000 characters of the inflated content, is there a way to do a "partial inflate" so I don't have to incur the full overhead of a total-file inflation? I know this seems like a long-shot, but I figured I'd ask for ideas.

MFN

In reply to Re^4: string pattern match, limited to first 1000 characters? by ManFromNeptune
in thread string pattern match, limited to first 1000 characters? by ManFromNeptune

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