Thread: Help with regular expressions

i'm trying develop better understanding of command line working way through book "the linux command line" schotts, available view here

http://linuxcommand.org/tlcl.php

i'm on section dealing regular expressions on page 262. uses example regular expression "only match lines consisting of groups of 1 or more alphabetic characters separated single spaces:"

code:

^([[:alpha:]]+ ?)+$

as example uses grep command

code:

carranty@carranty-desktop ~ $ echo "this that" | grep -e '^([[:alpha:]]+ ?)+$' that

the problem i'm having + command seems not work. same result if leave out

code:

carranty@carranty-desktop ~ $ echo "this that" | grep -e '^([[:alpha:]] ?)+$'  that

and want match lines consisting of 1 alphanumeric character followed space. think

code:

^([[:alpha:]]? ?)+$

would job (i've replaced + ?), doesn't

code:

carranty@carranty-desktop ~ $ echo "this that" | grep -e '^([[:alpha:]]? ?)+$' that

it still returns line though has more 1 character before space.

weirdly, third quantifier (the first ? in original code) works because if pass line more 1 space

code:

carranty@carranty-desktop ~ $ echo "this  that" | grep -e '^([[:alpha:]]+ ?)+$'  carranty@carranty-desktop ~ $

it doesn't output line. can please me out , explain i'm doing wrong.

ok, above post may little long heres more basic example. want match lines consisting of more 1 character. shouldn't

code:

[[:alpha:]]*

do it?? because isn't working me

code:

carranty@carranty-desktop ~ $ echo "t" | grep -e '[[:alpha:]]*' t

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