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Old 10-14-2007, 09:59 PM   #1
new_2_unix
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Registered: Oct 2007
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awk does not seem to recognize character classes


hi,

i have a text file with alphabets and numbers, as follows:

TestString123
AnotherString456

i'm using awk's "sub" function to substitute all alphabets with a "" so that i'm left with just the numbers, as follows:

cat filename | gawk '{sub(/TestString/,"")} ; { print $1 }'

This works just fine for the first line of the file and i've the output:

123
AnotherString456

My problem is that if i try to use character class [:alpha:] as follows:

cat filename | gawk '{sub(/[[:alpha:]]/,"")} ; { print $1 }'

it does not work properly and it removes only a part of the alphabets and not all, like the following output:

estString123
notherString456

if i try:
cat filename | gawk '{sub(/[A-Za-z]/,"")} ; { print $1 }'

that also does not work properly:

ing123
ing456

i can't figure out why the character classes or A-Za-z are only partially working whereas if i spell out the word explicitly, it works.

any guidance will be really helpful. thanks!
 
Old 10-14-2007, 10:04 PM   #2
matthewg42
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You want to use gsub. sub only substitutes the first occurrence, gsub (the g stands for global) replaces all occurrences.
 
Old 10-14-2007, 10:32 PM   #3
new_2_unix
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Registered: Oct 2007
Posts: 26

Original Poster
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Yes! That was it!
Thanks a lot!!
 
Old 10-14-2007, 10:33 PM   #4
Tinkster
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Quote:
Originally Posted by new_2_unix View Post
hi,

i have a text file with alphabets and numbers, as follows:

TestString123
AnotherString456

i'm using awk's "sub" function to substitute all alphabets with a "" so that i'm left with just the numbers, as follows:

cat filename | gawk '{sub(/TestString/,"")} ; { print $1 }'

This works just fine for the first line of the file and i've the output:

123
AnotherString456

My problem is that if i try to use character class [:alpha:] as follows:

cat filename | gawk '{sub(/[[:alpha:]]/,"")} ; { print $1 }'

it does not work properly and it removes only a part of the alphabets and not all, like the following output:

estString123
notherString456

if i try:
cat filename | gawk '{sub(/[A-Za-z]/,"")} ; { print $1 }'

that also does not work properly:

ing123
ing456

i can't figure out why the character classes or A-Za-z are only partially working whereas if i spell out the word explicitly, it works.

any guidance will be really helpful. thanks!
Works as defined. You're asking for ONE occurrence to be
removed, and that's what it's doing.

If you want ALL alphas to disappear, use a +
Code:
 gawk '{sub(/[[:alpha:]]+/,"")} ; { print $1 }' blahblah 
123
456
Same with [A-Za-z] ... w/o a + (or *) it will match ONE
(exactly one) character...


Cheers,
Tink
 
Old 10-14-2007, 10:41 PM   #5
new_2_unix
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Registered: Oct 2007
Posts: 26

Original Poster
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thanks for that too!
i didn't know that... gud thing to learn!
 
Old 10-14-2007, 11:13 PM   #6
Tinkster
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Quote:
Originally Posted by new_2_unix View Post
Yes! That was it!
Thanks a lot!!
Actually in this context sub does exactly what you
want. gsub would come in handy if the strings were
something like:

Code:
TestString123AnotherString456
and your expected result should be 123456... I'd
say that using gsub in the way described above is
abuse :}


Cheers,
Tink
 
Old 10-15-2007, 06:36 AM   #7
Geist3
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I find sed easier for substitutions:

echo st1906ghFU22 > teststring
sed 's/[[:alpha:]]//g' teststring
190622
 
  


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