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+ - Match An Element One Or More Times
The + metacharacter works much like the *, except it requires at least one instance of the preceding element to cause a match. Here is a regular expression that will only match lines consisting of groups of one or more alphabetic characters separated by single spa- ces:
^([[:alpha:]]+ ?)+$
[me@linuxbox ~]$ echo "This that" | grep -E '^([[:alpha:]]+ ?)+$'
This that
[me@linuxbox ~]$ echo "a b c" | grep -E '^([[:alpha:]]+ ?)+$'
a b c
[me@linuxbox ~]$ echo "a b 9" | grep -E '^([[:alpha:]]+ ?)+$'
[me@linuxbox ~]$ echo "abc d" | grep -E '^([[:alpha:]]+ ?)+$'
[me@linuxbox ~]$
[me@linuxbox ~]$ echo "This that" | grep -E '^([[:alpha:]]+ ?)+$'
This that
[me@linuxbox ~]$ echo "a b c" | grep -E '^([[:alpha:]]+ ?)+$'
a b c
[me@linuxbox ~]$ echo "a b 9" | grep -E '^([[:alpha:]]+ ?)+$'
[me@linuxbox ~]$ echo "abc d" | grep -E '^([[:alpha:]]+ ?)+$'
[me@linuxbox ~]$
We see that this expression does not match the line “a b 9”, because it contains a non-al- phabetic character; nor does it match “abc d”, because more than one space character separates the characters “c” and “d”.