if (eregi('^(ab|)c$',$x)) print "blah: $x<br>\n";

complains with 
Warning: REG_EMPTY in /usr/local/ananova_v2/www/search/_index.php on line 31

and returns false regardless of $x

yet:

echo "c" | egrep '^(ab|)c$'
echo "ac" | egrep '^(ab|)c$'
echo "abc" | egrep '^(ab|)c$'

outputs:
c
abc

Removing the from my php expression | stops the complaint but ruins the pattern.

My pattern intends to read (in english)
Optionally prefixed by "ab" and matches "c" exactly.