The regex engine has many optimizations and heuristics to speed things up, you are just falling back to "normal" speed.

> Is there some weirdness with alternations and anchors?

It's a feature called "trie optimization"¹ which only works for literal characters and can't optimize meta symbols like anchors.

> (I could rewrite ^aaa|^bbb|^ccc as ^(aaa|bbb|ccc) , but it might be that only some of the inputs are anchored.)

So make two or-lists one with preceding anchor, one without and or them. Both will be trie-optimized and this should be max speed again.

Like:

((?:^(?:aaa|bbb|ccc))|(?:ddd|eee|ccc))

*untested!!!*

(?:...) is for grouping without matching capturing.

(Not sure if the one around the anchored part is necessary... I don't think so, just playing safe)

Cheers Rolf
(addicted to the Perl Programming Language :)
see Wikisyntax for the Monastery

¹) compare Re^2: Looking for a cleaner regex ( trie since 5.10 ! )

Update

Tested in perldebugger

perl -de0

...
  DB<11> @matches = ( "aaa x ddd y ccc" =~ /(^ (?:aaa|bbb|ccc) | (?:dd
+d|eee|ccc) )/xg ); 

  DB<12> print "@matches"
aaa ddd ccc
  DB<13>

[download]

In reply to Re: Alternations and anchors (trie optimization) by LanX
in thread Alternations and anchors by Chuma

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