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What is the meaning of this awk expression using sub?

 
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# 1  
Old 01-19-2012
What is the meaning of this awk expression using sub?
Cannot understand what the "&" in the sub expression is supposed to do.

Code:
{sub(/^/,"&" s)}

# 2  
Old 01-19-2012
It does a search/replace in the current line( $0 ), stopping after the first replace.

The regular expression /^/ is what's being searched for. ^ has a special meaning in expressions, "the beginning of the line".

It replaces it with the literal string "&" S. So what it actually puts in there depends on what value S holds.

So if S was "asdf", this command would add "&asdf" to the beginning of the line.
# 3  
Old 01-19-2012
What would be the difference between these two expressions? They seem to do the same.

Code:
{sub(/^/, s "&")}

Code:
{sub(/^/, "&" s)}

---------- Post updated at 04:11 PM ---------- Previous update was at 04:04 PM ----------

Quote:
Originally Posted by Corona688
It does a search/replace in the current line( $0 ), stopping after the first replace.

The regular expression /^/ is what's being searched for. ^ has a special meaning in expressions, "the beginning of the line".

It replaces it with the literal string "&" S. So what it actually puts in there depends on what value S holds.

So if S was "asdf", this command would add "&asdf" to the beginning of the line.

It seems that & prints the entire text when used in a replacement string.

---------- Post updated at 04:12 PM ---------- Previous update was at 04:11 PM ----------

I have done like this and the results are shown. Not exactly what you are saying.
Code:
echo "Hello" | awk '{s="test";sub ( /^/, "&" s );print}'
testHello

echo "Hello" | awk '{s="test";sub ( /^/, "&" s );print}'
testHello

Also, the one below gives also same result

Code:
echo "Hello" | awk '{s="test";sub ( /^/, s );print}'
testHello

# 4  
Old 01-19-2012
That is very mysterious. I have no idea why the ampersand is being dropped, though it does it for me too.

If you use anything but ampersand it behaves as I described.

---------- Post updated at 03:21 PM ---------- Previous update was at 03:18 PM ----------

Ahah:

Code:
       gsub(r, s [, t])        For each substring matching the regular expres-
                               sion r in the string t, substitute  the  string
                               s,  and return the number of substitutions.  If
                               t is  not  supplied,  use  $0.   An  &  in  the
                               replacement text is replaced with the text that
                               was actually matched.  Use \& to get a  literal
                               &.   (This  must  be  typed as "\\&"; see GAWK:
                               Effective AWK Programming for a fuller  discus-
                               sion  of  the  rules for &'s and backslashes in
                               the replacement text of sub(), gsub(), and gen-
                               sub().)

gsub works the same as sub except it does multiple matches.

Since /^/ doesn't actually match any text, "&" becomes nothing here.
This User Gave Thanks to Corona688 For This Post:
kristinu
 
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