Regular Expression

Regular Expression format: special character + normal character

1. find express which start with FROM or SUBJECT: 

^(FROM|SUBJECT): 

2. replace <emphasis>ip address</emphasis> to <inet>ip address</inet>

(perl) s!<emphasis>([0-9]+(\.[0-9]+){3})</emphasis>!<inet>$1</inet>!

Special Character

1. ^: start of line

2. $: end of line

^$: match empty line

^cat: match line start with "cat"

cat$: match line end with "cat"

^cat$: match line only contains "cat"

3. []: character group

A. [0-9]: match all number

B. [a-z]: match all lower letter

C. [A-Z]: match all upper letter

D. [0-9A-Za-z]: match all number, lower letter and upper letter

gr[ae]y --> gray / grey

<H[123]> --> <H1> / <H2> / <H3>

<H[1-3]> --> <H1> / <H2> / <H3>

4. [^]: get rid of character group

[^1-6]: no match 1~6;

5. . : match one any item

03.19.76 --> 03/19/76; 30-19-76

6. |: or

gr(a|e)y --> gray; grey

Bob|Robert --> Bob; Robert

7. \b: separate words

\bcat\b: match "cat" word

8.{}: match duplicate times

[a-zA-Z]{1,5}: match stock code 1 to 5 letter

? == {0,1}

9. \: match following word not special character

10. match: =~//

Example:

print "Enter a temperature in Celsius: \n";

$celsius=<STDIN>;

chomp($celsius);

if($celsius=~m/^[0-9]+$/)

{

    $fahrenheit=($celsius*9/5)+32;

    print "$celsius C is $fahrenheit F. \n"

}

else

{

    print "Expecting a number."

}

perl regular expression

\t --> tab

\n --> new line

\r --> enter

\s --> any empty space

\S --> any character except \s

\w --> [a-zA-Z0-9]

\W --> any character except \w

\d--> [0-9]

\D --> any character except \d