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

==> See the explanation and example here

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

==> See the explanation and example here

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

==> See the explanation and example here

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

==> See the explanation and example here

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 sensitive checking
( # 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

==> See the explanation and example here

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

==> See the explanation and example here

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] # follow 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

==> See the explanation and example here

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] # follow by 0..5 and 0..9, which means 00 to 59

==> See the explanation and example here

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

==> See the explanation and example here

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 ">"

==> See the explanation and example here

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 "<a"
( # start of group #2
[^>]+ # anything except (">"), at least one character
) # end of group #2
> # follow by ">"
(.+?) # match anything
</a> # end with "</a>

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 / double quotes and ">"
) # end of group #1

==> See the explanation and example here

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