Quoth Anonymous on Thursday, 12 August 2010: > Oliver Fromme <o...@lurza.secnetix.de> writes: > > > John Levine <jo...@iecc.com> wrote: > > > > > % egrep 'word1|word2|word3|...|wordn' filename.txt > > > > > > > Thanks for the replies. This suggestion won't do the job as the list of > > > > words is very long, maybe 50-60. This is why I asked how to place them > > all > > > > in a file. One reply dealt with using a file with egrep. I'll try that. > > > > > > Gee, 50 words, that's about a 300 character pattern, that's not a problem > > > for any shell or version of grep I know. > > > > > > But reading the words from a file is equivalent and as you note most > > > likely easier to do. > > > > The question is what is more efficient. This might be > > important if that kind of grep command is run very often > > by a script, or if it's run on very large files. > > > > My guess is that one large regular expression is more > > efficient than many small ones. But I haven't done real > > benchmarks to prove this. > > BTW, not using regular expressions is even more efficient, e.g. > > $ fgrep -f /usr/share/dict/words /etc/group > > When using egrep(1) it takes considerably more time and memory.
Having written a regex engine myself, I can see why. Though I'm sure egrep is highly optimized, even the most optimized DFA table is going to take more cycles to navigate than a simple string comparison. Not to mention the initial overhead of parsing the regex and building that table. -- Sterling (Chip) Camden | sterl...@camdensoftware.com | 2048D/3A978E4F http://camdensoftware.com | http://chipstips.com | http://chipsquips.com
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