Showing posts with label pattern match. Show all posts

Look ahead and Look behind in perl

With the look-ahead and look-behind constructs ,you can "roll your own" zero-width assertions to fit your needs. You can look forward or backward in the string being processed, and you can require that a pattern match succeed (positive assertion) or fail (negative assertion) there.
Every extended pattern is written as a parenthetical group with a question mark as the first character. The notation for the look-arounds is fairly mnemonic, but there are some other, experimental patterns that are similar, so it is important to get all the characters in the right order. 
(?=pattern)
is a positive look-ahead assertion 
(?!pattern)
is a negative look-ahead assertion 
(?<=pattern)
is a positive look-behind assertion 
(?<!pattern)
is a negative look-behind assertion
EXAMPLES
Look-Ahead: 
echo $mytmp2
uvw_abc uvw_def uvw_acb

Positive:

echo $mytmp2 | perl -pe 's/uvw_(?=(abc|def))/xyz_/g'
xyz_abc xyz_def uvw_acb
Description: replace every occurance of uvw_ with xyz_ where uvw_ followed by abc or def 
Negative:

echo $mytmp2 | perl -pe 's/uvw_(?!(abc|def))/xyz_/g'
uvw_abc uvw_def xyz_acb
Description: replace every occurance of uvw_ with xyz_ where uvw_ is not followed by abc or def
Look-Behind:

echo $mytmp
abc_uvw def_uvw acb_uvw

Positive:

echo $mytmp | perl -pe 's/(?<=(abc|def))_uvw/_xyz/g'
abc_xyz def_xyz acb_uvw
Description: replace every occurance of _uvw with _xyz where _uvw is preceeded by abc or def
Negative:

echo $mytmp | perl -pe 's/(?<!(abc|def))_uvw/_xyz/g'
abc_uvw def_uvw acb_xyz
Description: replace every occurance of _uvw with _xyz where _uvw is not preceeded by abc or def
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Pattern Match Using Perl-Append file contents

If i want to append all the lines of one file  to a line in second file only if there is a pattern match:

For example:
first_file.txt:
111111
1111
11
1

second_file.txt:
122221
2222
22
2

pattern to match:
2222
output:

122221
111111
1111
11
1
2222
111111
1111
11
1
22
2

Below is the perl command to achieve this:

perl -lne 'BEGIN{open(A,"first_file.txt");@f=<A>}
           print;if(/2222/){print @f}' 
           second_file.txt
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Capture all the letters which are repeated more than once

Recently i came across a need where i need to fetch all the letters in a line which are repeated more than once in a line contiguously.

for example :

lets say there a word "foo bar". I want the letter 'o' in this.
lets say there a word "foo baaar". I want the letters 'o','a' in this.
lets say there a word "foo baaar foo". I want the letters 'o','a' in this again.

Below is code which worked for me:

perl -lne 'push @a,/(\w)\1+/g;END{print @a}' your_file

The above command will scan complete file and prints just the letters that are repeated continguously in the file.
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