Wednesday, February 12, 2014

10 Java Regular Expression Examples You Should Know

10 Java Regular Expression Examples You Should Know

Regular expression is an art of the programing, it’s hard to debug , learn and understand, but the powerful features are still attract many developers to code regular expression. Let’s explore the following 10 practical regular expression ~ enjoy
1. Username Regular Expression Pattern
^[a-z0-9_-]{3,15}$
^                    # Start of the line
 [a-z0-9_-]     # Match characters and symbols in the list, a-z, 0-9 , underscore , hyphen
            {3,15}  # Length at least 3 characters and maximum length of 15
$                    # End of the line
2. Password Regular Expression Pattern
((?=.*\d)(?=.*[a-z])(?=.*[A-Z])(?=.*[@#$%]).{6,20})
( # Start of group
 (?=.*\d) #   must contains one digit from 0-9
 (?=.*[a-z]) #   must contains one lowercase characters
 (?=.*[A-Z]) #   must contains one uppercase characters
 (?=.*[@#$%]) #   must contains one special symbols in the list "@#$%"
             . #     match anything with previous condition checking
               {6,20} #        length at least 6 characters and maximum of 20   
) # End of group
3. Hexadecimal Color Code Regular Expression Pattern
^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$
^ #start of the line
# #  must constains a "#" symbols
( #  start of group #1
 [A-Fa-f0-9]{6} #    any strings in the list, with length of 6
 | #    ..or
 [A-Fa-f0-9]{3} #    any strings in the list, with length of 3
) #  end of group #1
$ #end of the line
4. Email Regular Expression Pattern
^[_A-Za-z0-9-]+(\\.[_A-Za-z0-9-]+)*@[A-Za-z0-9]+(\\.[A-Za-z0-9]+)*(\\.[A-Za-z]{2,})$
^ #start of the line
 [_A-Za-z0-9-]+ #  must start with string in the bracket [ ], must contains one or more (+)
 ( #  start of group #1
   \\.[_A-Za-z0-9-]+ #     follow by a dot "." and string in the bracket [ ], must contains one or more (+)
 )* #  end of group #1, this group is optional (*)
   @ #     must contains a "@" symbol
    [A-Za-z0-9]+       #        follow by string in the bracket [ ], must contains one or more (+)
     ( #   start of group #2 - first level TLD checking
      \\.[A-Za-z0-9]+  #     follow by a dot "." and string in the bracket [ ], must contains one or more (+)
     )* #   end of group #2, this group is optional (*)
     ( #   start of group #3 - second level TLD checking
      \\.[A-Za-z]{2,}  #     follow by a dot "." and string in the bracket [ ], with minimum length of 2
     ) #   end of group #3
$ #end of the line
5. Image File Extension Regular Expression Pattern
([^\s]+(\.(?i)(jpg|png|gif|bmp))$)
( #Start of the group #1
[^\s]+ #  must contains one or more anything (except white space)
      ( #    start of the group #2
        \. # follow by a dot "."
        (?i) # ignore the case sensive checking for the following characters
            ( #  start of the group #3
             jpg #    contains characters "jpg"
             | #    ..or
             png #    contains characters "png"
             | #    ..or
             gif #    contains characters "gif"
             | #    ..or
             bmp #    contains characters "bmp"
            ) #  end of the group #3
      ) #     end of the group #2
 $ #  end of the string
) #end of the group #1
6. IP Address Regular Expression Pattern
^([01]?\\d\\d?|2[0-4]\\d|25[0-5])\\.([01]?\\d\\d?|2[0-4]\\d|25[0-5])\\.
([01]?\\d\\d?|2[0-4]\\d|25[0-5])\\.([01]?\\d\\d?|2[0-4]\\d|25[0-5])$
^ #start of the line
( #  start of group #1
  [01]?\\d\\d? #    Can be one or two digits. If three digits appear, it must start either 0 or 1
#    e.g ([0-9], [0-9][0-9],[0-1][0-9][0-9])
   | #    ...or
  2[0-4]\\d #    start with 2, follow by 0-4 and end with any digit (2[0-4][0-9])
   |           #    ...or
  25[0-5]      #    start with 2, follow by 5 and end with 0-5 (25[0-5])
) #  end of group #2
 \.            #  follow by a dot "."
....            # repeat with 3 time (3x)
$ #end of the line
7. Time Format Regular Expression Pattern
Time in 12-Hour Format Regular Expression Pattern
(1[012]|[1-9]):[0-5][0-9](\\s)?(?i)(am|pm)
( #start of group #1
1[012] #  start with 10, 11, 12
| #  or
[1-9] #  start with 1,2,...9
) #end of group #1
: #    follow by a semi colon (:)
 [0-5][0-9] #      follw by 0..5 and 0..9, which means 00 to 59
           (\\s)? #        follow by a white space (optional)
                 (?i) #          next checking is case insensitive
                     (am|pm) #            follow by am or pm
Time in 24-Hour Format Regular Expression Pattern
([01]?[0-9]|2[0-3]):[0-5][0-9]
( #start of group #1
[01]?[0-9] #  start with 0-9,1-9,00-09,10-19
| #  or
2[0-3] #  start with 20-23
) #end of group #1
: #    follow by a semi colon (:)
 [0-5][0-9] #      follw by 0..5 and 0..9, which means 00 to 59
8. Date Format (dd/mm/yyyy) Regular Expression Pattern
(0?[1-9]|[12][0-9]|3[01])/(0?[1-9]|1[012])/((19|20)\\d\\d)
( #start of group #1
0?[1-9] #  01-09 or 1-9
|                   #  ..or
[12][0-9] #  10-19 or 20-29
| #  ..or
3[01] #  30, 31
) #end of group #1
 / #  follow by a "/"
  ( #    start of group #2
   0?[1-9] # 01-09 or 1-9
   | # ..or
   1[012] # 10,11,12
   ) #    end of group #2
    / # follow by a "/"
     ( #  start of group #3
      (19|20)\\d\\d #    19[0-9][0-9] or 20[0-9][0-9]
      ) #  end of group #3
9. HTML tag Regular Expression Pattern
<("[^"]*"|'[^']*'|[^'">])*>
<   #start with opening tag "<"
( #   start of group #1
  "[^"]*" # only two double quotes are allow - "string"
  | # ..or
  '[^']*' # only two single quotes are allow - 'string'
  | # ..or
  [^'">] # cant contains one single quotes, double quotes and ">"
) #   end of group #1
* # 0 or more
> #end with closing tag ">"
10. HTML links Regular Expression Pattern
HTML A tag Regular Expression Pattern
(?i)<a([^>]+)>(.+?)</a>
( #start of group #1
?i #  all checking are case insensive
) #end of group #1
<a              #start with "
 ( #  start of group #2
   [^>]+ #     anything except (">"), at least one character
  ) #  end of group #2
 > #     follow by ">"
   (.+?) # match anything
        </a> #  end with "

Extract HTML link Regular Expression Pattern
\s*(?i)href\s*=\s*(\"([^"]*\")|'[^']*'|([^'">\s]+));
\s*   #can start with whitespace
 (?i)   # all checking are case insensive
    href   #  follow by "href" word
       \s*=\s*   #   allows spaces on either side of the equal sign,
             (   #    start of group #1
              "([^"]*")   #      only two double quotes are allow - "string"
              |   #  ..or
              '[^']*'   #      only two single quotes are allow - 'string'
              |           #  ..or
              ([^'">]+)   #      cant contains one single quotes, double quotes ">"
     )   #    end of group #1

Want to learn more about regular expression? Highly recommend the best and classic book – “Mastering Regular Expression”

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