Since I keep forgetting this – especially because I cannot remember the “why”: [WayBack] shell – How do I grep for multiple patterns with pattern having a pipe character? – Unix & Linux Stack Exchange by “user unknown“.
The -E means using Regular expression: POSIX extended – Wikipedia.
egrep "foo|bar" *.txtor
grep "foo\|bar" *.txt grep -E "foo|bar" *.txtselectively citing the man page of gnu-grep:
-E, --extended-regexp Interpret PATTERN as an extended regular expression (ERE, see below). (-E is specified by POSIX.) Matching Control -e PATTERN, --regexp=PATTERN Use PATTERN as the pattern. This can be used to specify multiple search patterns, or to protect a pattern beginning with a hyphen (-). (-e is specified by POSIX.)(…)
grep understands two different versions of regular expression syntax: “basic” and “extended.” In GNU grep, there is no difference in available functionality using either syntax. In other implementations, basic regular expressions are less powerful. The following description applies to extended regular expressions; differences for basic regular expressions are summarized afterwards.In the beginning I didn’t read further, so I didn’t recognize the subtle differences:
Basic vs Extended Regular Expressions In basic regular expressions the meta-characters ?, +, {, |, (, and ) lose their special meaning; instead use the backslashed versions \?, \+, \{, \|, \(, and \).I always used egrep and needlessly parens, because I learned from examples. Now I learned something new. :)
–jeroen
The Wiert Corner - irregular stream of stuff 