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Capturing group difference in Basic Regular Expressions (BRE) and Extended Regular Expressions (ERE)

Jun 222010

Capturing group difference in Basic Regular Expressions (BRE) and Extended Regular Expressions (ERE)

Answer:

One of the very confusion issue related to most GNU/Linux utils which are using regular expression library is they are mostly supporting the Basic Regular Expressions (BRE) by default.

E.g.

# echo "(foo)" | sed 's/$foo$/bar/gi'

You might expect the result is

bar

But the actual result is...

(bar)

Why the brackets are not replaced? It is because in BRE (which is the default, you don't need to escape, if you escape it, then it will be treated as a capturing group - which is quite non-sense if you came from programming background).

To solve the problem, you can just simply remove the \ escape, or you tell those commands to use ERE, e.g.

# echo "(foo)" | sed -r 's/\(foo\)/bar/gi'
bar

Replace infile using sed and make a backup automatically

Advanced Linux No Responses »

Jun 212010

Replace infile using sed and make a backup automatically

Answer:

If you use the -i flag in sed, it will replace infile, which is quite dangerous if you don't have a backup.

E.g.

# sed -i 's/foo/bar/gi' input.txt

To auto make a backup when replacing in a file, you can use

# sed -i.bak 's/foo/bar/gi' input.txt

Now the original file will stored as input.txt.bak in the same directory that you are working.

Remove the last character of a string in Perl

Perl No Responses »

Jun 202010

Remove the last character of a string in Perl

Answer:

If you want to remove the last character of a string in Perl, e.g.

abc => ab

You can use the chop() function.

my $string = 'abc';
chop($string);
print $string; # give ab

Get array size in Perl

Perl No Responses »

Jun 192010

Get array size in Perl

Answer:

To get the size of array in Perl, you have two methods.

Sample code:

my @a = (1, 2, ,3);

1. By scalar context

my $size = scalar @a;

2. By special variable $#array

my $size = $#a + 1;

Simple file encryption with OpenSSL

Basic Linux, Linux Security No Responses »

Jun 182010

Simple file encryption with OpenSSL

Answer:

You have a single file (input.txt), and you want to encrypt it with a password, it is easy with OpenSSL.

# openssl aes-128-cbc -salt -in input.txt-out input.aes

Enter the password twice.

When you want to decrypt it, use the command.

# openssl aes-128-cbc -d -salt -in input.aes -out output.txt

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