Bash parameter expansion

Have you ever seen ## (hash hash/pound pound) or %% (percent percent) inside a bash script and wondered what it means? It's a form of parameter expansion that allows you to manipulate your strings using regexes.

All the following commands work with a variable called $string.
'pattern' means bash pattern, or can also be an ordinary string:

• String length: ${#string}
• Extract a substring: ${string:position}
• Extract a substring, specifying length: ${string:position:length}
• Delete shortest match of substring from the beginning of string: ${string#substring}
• Delete shortest match of substring from the end of string: ${string%substring}
• Delete longest match of substring from the beginning of string: ${string##substring}
• Delete longest match of substring from the end of string: ${string%%substring}
• Find and replace first substring: ${string/pattern/replacement}
• Find and replace all substrings: ${string//pattern/replacement}

Thanks to TheGeekStuff

Sprintf in Java

import java.text.MessageFormat;

int a = 3;
String b = "coders";

System.out.println(
    MessageFormat.format("{0} Java {1}.", String.format("%04d", a), b)
);

PHP arrays

This won't work:

php -r "$a = array( 'c' => array( 'b' => 1 ) );"

But this will:

php -r '$a = Array( "c" => Array( "b" => 1 ) );'

On Mac OSX

PHP regular expressions

 /(.*)something/s

The s at the end causes the dot to match all characters including newlines.

Use a string to make a variable name

my $rank10 = 42;
{
    no strict 'refs';
    my $num = 10;
    print eval "\$rank$num";
}

result:
    42

Perl sprintf

        # Format number with up to 8 leading zeroes
        $result = sprintf("%08d", $number);

        # Round number to 3 digits after decimal point
        $rounded = sprintf("%.3f", $number);

Perl's sprintf permits the following universally-known conversions:

   %% a percent sign
 %c a character with the given number
 %s a string
 %d a signed integer, in decimal
 %u an unsigned integer, in decimal
 %o an unsigned integer, in octal
 %x an unsigned integer, in hexadecimal
 %e a floating-point number, in scientific notation
 %f a floating-point number, in fixed decimal notation
 %g a floating-point number, in %e or %f notation

Regular expressions in C: Advice

If you can avoid using regular expressions in C, then *do not use them!!*

Instead try the strstr (find substring) and strtok (like perl's split) routines.

Comparing strings in C

In c it is not possible to directly compare two strings so a statement like if (string1==string2) is not valid. Most c libraries contain a function called the strcmp().This is used to compare two strings in the following manner.

if(strcmp(name1,name2)==0)
 puts("The names are the same");
else
 puts("The names are not the same.");

Thanks johnt