[SOLVED] Discuss about regexp ...

[Mainsun]

Hi all,
Accounting to gnu/sed manual,
http://www.gnu.org/software/sed/manu...ar-Expressions
*
Matches a sequence of zero or more instances of matches for the preceding regular expression, which must be an ordinary character, a special character preceded by \, a ., a grouped regexp (see below), or a bracket expression. As a GNU extension, a postfixed regular expression can also be followed by *; for example, a** is equivalent to a*. POSIX 1003.1-2001 says that * stands for itself when it appears at the start of a regular expression or subexpression, but many nonGNU implementations do not support this and portable scripts should instead use \* in these contexts.
In my test.txt

ab
aab
abb
a1234b
a???b
a b
a is not b
a
b

I want to replace all a*b to cd. So I run.

sed 's/a*b/cd/g' test.txt

However, the output is not expected!

cd
cd
cdcd
a cd
a is not cd
a
cd

Can someone state what 's wrong?

[Adunaic]

Perhaps you could post what the output is you were expecting.

* matches to zero or more instances of teh previous character / expression.

from your expression

sed 's/a*b/cd/g' test.txt

valid matches would include:

b (as zero a's)
ab (as one a)
aab (as more than one a).

ab   (matches as there is 1 a followed by a b. replaced with cd)
aab (more than one a followed by a b so replaced with cd)
abb (ab is matched and changed to cd. this leaves b which matches so is replaced by cd as well.)
a1234b (b is preceeded by zero a's so changed to cd)
a???b (b is preceeded by zero a's so changed to cd)
a b (b is preceeded by zero a's so changed to cd)
a is not b (b is preceeded by zero a's so changed to cd)
a (no match, must be followed by a b)
b (b is preceeded by zero a's so changed to cd)

Not sure why you did not get a match for lines 4 and 5. but it is doing what you want.

If you want it to match anything surrouned by a n 'a' and a 'b' you need to use:

a.*b

where the .* be means any and as many characters as there are.

[Mainsun]

thanks Adunaic ^_^.
your explanation is so clear!