# Common Regular Expression patterns Common string formats used in software development and examples of regular expressions to check their correctness. ## Username Regular Expression Pattern A 8 to 24 character passwords that can include any lower case character or digit (number). Only the underscore and dash special characters can be used. ```clojure (re-matches #"^[a-z0-9_-]{8,24}$" "good-username") ``` Breakdown the regex pattern: ```regex ^[a-z0-9_-]{8,24}$ ^ # Start of the line [a-z0-9_-] # Match characters and symbols in the list, a-z, 0-9 , underscore , hyphen {8,24} # Length at least 8 characters and maximum length of 24 $ # End of the line ``` ## Password Regular Expression Pattern A password should be 8 to 24 character string with at least one digit, one upper case letter, one lower case letter and one special symbol, `@#$%`. ```clojure (re-matches #"((?=.*\d)(?=.*[a-z])(?=.*[A-Z])(?=.*[@#$%]).{8,24})" "G00d @ username") ``` The order of the grouping formulas does not matter Breakdown the regex pattern: ```regex ((?=.*\d)(?=.*[a-z])(?=.*[A-Z])(?=.*[@#$%]).{8,24}) ( # Start of group (?=.*\d) # must contains one digit from 0-9 (?=.*[a-z]) # must contains one lowercase characters (?=.*[A-Z]) # must contains one uppercase characters (?=.*[@#$%]) # must contains one special symbols in the list "@#$%" . # match anything with previous condition checking {8,24} # length at least 8 characters and maximum of 24 ) # End of group ``` `?=` means apply the assertion condition, which is meaningless by itself and works in combination with others. ## Hexadecimal Color Code Regular Expression Pattern The string must start with a `#`symbol , follow by a letter from `a` to `f`, `A` to `Z` or a digit from `0` to `9` with a length of exactly 3 or 6.` This regular expression pattern is very useful for the Hexadecimal web colors code checking. ```clojure (re-matches #"^#([A-Fa-f0-9]{3}|[A-Fa-f0-9]{6})$" "#FFAABB") ``` Breakdown the regex pattern: ```regex ^#([A-Fa-f0-9]{3}|[A-Fa-f0-9]{6})$ ^ #start of the line # # must contain a "#" symbols ( # start of group #1 [A-Fa-f0-9]{3} # any strings in the list, with length of 3 | # ..or [A-Fa-f0-9]{6} # any strings in the list, with length of 6 ) # end of group #1 $ #end of the line ``` ## Email Regular Expression Pattern The account side of an email address starts with `_A-Za-z0-9-\\+` optional follow by `.[_A-Za-z0-9-]`, ending with an `@` symbol. The domain starts with `A-Za-z0-9-`, follow by first level domain, e.g `.org`, `.io` and `.[A-Za-z0-9]` optionally follow by a second level domain, e.g. `.ac.uk`, `.com.au` or `\\.[A-Za-z]{2,}`, where second level domain must start with a dot `.` and length must equal or more than 2 characters. ```clojure (re-matches #"^[_A-Za-z0-9-]+(\.[_A-Za-z0-9-]+)*@[A-Za-z0-9]+(\.[A-Za-z0-9]+)*(\.[A-Za-z]{2,})$" "jenny.jenn@jetpack.com.au") ``` !!! HINT "Double escaping special characters" Double escaping of special characters is not required in the Clojure syntax. Breakdown the regex pattern: ```regex ^[_A-Za-z0-9-]+(\\.[_A-Za-z0-9-]+)*@[A-Za-z0-9]+(\\.[A-Za-z0-9]+)*(\\.[A-Za-z]{2,})$ ^ #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 ``` ## Image File name and Extension Regular Expression Pattern A file extension name is 1 or more characters without white space, follow by dot `.` and string end in `jpg` or `png` or `gif` or `bmp`. The file name extension is case-insensitive. Change the combination `(jpg|png|gif|bmp)` for other file extension. ```clojure (re-matches #"(?i)([^\s]+(\.(jpg|png|gif|bmp))$)" "clojure-logo.png") ``` ??? HINT "In-line modifiers indirectly supported in ClojureScript" ClojureScript is hosted on JavaScript which does not support in-line modifier flags such as `(?i)` for a case insensitive pattern. In-line flags will be [converted by the ClojureScript reader if they are the first element in the literal regular expression pattern](https://stackoverflow.com/a/23187290/1762872), or if the `js/RegExp` function is used to create the regular expression. Breakdown the regex pattern: ```regex ([^\s]+(\.(?i)(jpg|png|gif|bmp))$) ( #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 ``` ## IP Address Regular Expression Pattern An IP address comprises of 4 groups of numbers between 0 and 255, with each group separated by a dot. Example IP address are: `192.168.0.1`, `127.0.0.1`, `192.120.240.100` ```clojure (re-matches #"^([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])$" "192.168.0.1") ``` Breakdown the regex pattern: ```regex ^([01]?\\d\\d?|2[0-4]\\d|25[0-5])\\.([01]?\\d\\d?|2[0-4]\\d|25[0-5])\\. ([01]?\\d\\d?|2[0-4]\\d|25[0-5])\\.([01]?\\d\\d?|2[0-4]\\d|25[0-5])$ ^ #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 ``` ## Time Format Regular Expression Pattern Time in 12-Hour Format Regular Expression Pattern. The 12-hour clock format start between 0-12, then a semi colon, `:`, follow by `00-59`. The pattern ends with `am` or `pm`. ```clojure (re-matches #"(?i)(1[012]|[1-9]):[0-5][0-9](\s)?(am|pm)" "12:59am") ``` Breakdown the regex pattern: ```regex (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 ``` ## Time in 24-Hour Format Regular Expression Pattern The 24-hour clock format start between 0-23 or 00-23, then a semi colon `:` and follow by 00-59. ```clojure (re-matches #"(([01]?[0-9]|2[0-3]):[0-5][0-9])" "23:58") ``` Breakdown the regex pattern: ```regex ([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 ``` ## Date Format Pattern Date format in the form `dd/mm/yyyy`. Validating a leap year and if there is 30 or 31 days in a month is not simple though. ```clojure (re-matches #"(0?[1-9]|[12][0-9]|3[01])/(0?[1-9]|1[012])/((19|20)\d\d)" "20/02/2020") ``` Breakdown the regex pattern: ```regex (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 ``` ## HTML tag Pattern HTML code uses tags to define structure of content. HTML tag, start with an opening tag “<" , follow by double quotes "string", or single quotes 'string' but does not allow one double quotes (") "string, one single quote (') 'string or a closing tag > without single or double quotes enclosed. At last , end with a closing tag “>” ```clojure (re-matches #"<("[^"]*"|'[^']*'|[^'">])*>" "

Title

Loreum ipsum

") ``` Breakdown the regex pattern: ```regex <("[^"]*"|'[^']*'|[^'">])*> < #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 ">" ``` ## HTML links Regular Expression Pattern HTML A tag Regular Expression Pattern ```regex (?i)]+)>(.+?) ( #start of group #1 ?i # all checking are case insensitive ) #end of group #1 ]+ # anything except (">"), at least one character ) # end of group #2 > # follow by ">" (.+?) # match anything # end with " ``` ## Extract HTML link Regular Expression Pattern ```regex \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 ``` ## Reference * [10 Java regular expression examples](https://mkyong.com/regular-expressions/10-java-regular-expression-examples-you-should-know)