Regular expressions examples

Username Regular Expression Pattern

^[a-z0-9_-]{3,15}$

Description

^                    # 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

Whole combination is means, 3 to 15 characters with any lower case character, digit or special symbol “_-” only. This is common username pattern that’s widely use in different websites.

Image File Extension Regular Expression Pattern

([^\s]+(\.(?i)(jpg|png|gif|bmp))$)

Description

(			#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

Whole combination is means, must have 1 or more strings (but not white space), follow by dot “.” and string end in “jpg” or “png” or “gif” or “bmp” , and the file extensive is case-insensitive.

This regular expression pattern is widely use in for different file extensive checking. You can just change the end combination (jpg|png|gif|bmp) to come out different file extension checking that suit your need.

Time in 12-Hour Format Regular Expression Pattern

(1[012]|[1-9]):[0-5][0-9](\\s)?(?i)(am|pm)

Description

(				#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

The 12-hour clock format is start from 0-12, then a semi colon (:) and follow by 00-59 , and end with am or pm.