How to write Regular Expressions?

08 Jul, 2016
A regular expression (sometimes called a rational expression) is a sequence of characters that define a search pattern, mainly for use in pattern matching with strings, or string matching, i.e. “find and replace”-like operations.(Wikipedia).
Regular expressions are a generalized way to match patterns with sequences of characters. It is used in every programming language like C++, Java and Python.
What is a regular expression and what makes it so important?Regex are used in Google analytics in URL matching in supporting search and replace in most popular editors like Sublime, Notepad++, Brackets, Google Docs and Microsoft word.
Example :  Regular expression for an email address :
^([a-zA-Z0-9_\-\.]+)@([a-zA-Z0-9_\-\.]+)\.([a-zA-Z]{2,5})$ 

The above regular expression can be used for checking if a given set of characters is an email address or not.
How to write regular expression?
Repeaters : * , + and { } :These symbols act as repeaters and tell the computer that the preceding character is to be used for more than just one time.
The asterisk symbol ( * ):It tells the computer to match the preceding character (or set of characters) for 0 or more times (upto infinite).Example : The regular expression ab*c will give ac, abc, abbc, 
abbbc....ans so on 
Example : The regular expression ab*c will give ac, abc, abbc, 
abbbc....ans so on 
The Plus symbol ( + ):It tells the computer to repeat the preceding character (or set of characters) for atleast one or more times(upto infinite).Example : The regular expression ab+c will give abc, abbc,
abbc, ... and so on.
Example : The regular expression ab+c will give abc, abbc,
abbc, ... and so on.
The curly braces {...}:It tells the computer to repeat the preceding character (or set of characters) for as many times as the value inside this bracket.Example : {2} means that the preceding character is to be repeated 2 
times, {min,} means the preceding character is matches min or  more 
times. {min,max} means that the preceding character is repeated at
least min & at most max times.

Example : {2} means that the preceding character is to be repeated 2 
times, {min,} means the preceding character is matches min or  more 
times. {min,max} means that the preceding character is repeated at
least min & at most max times.

Wildcard – ( . )The dot symbol can take place of any other symbol, that is why itis called the wildcard character.Example : 
The Regular expression .* will tell the computer that any character
can be used any number of times.
Example : 
The Regular expression .* will tell the computer that any character
can be used any number of times.
Optional character – ( ? )This symbol tells the computer that the preceding character mayor may not be present in the string to be matched.Example : 
We may write the format for document file as – “docx?”
The ‘?’ tells the computer that x may or may not be 
present in the name of file format.
Example : 
We may write the format for document file as – “docx?”
The ‘?’ tells the computer that x may or may not be 
present in the name of file format.
The caret ( ^ ) symbol: Setting position for match :tells the computer that the match must start at the beginning of the string or line.Example : ^\d{3} will match with patterns like "901" in "901-333-".
Example : ^\d{3} will match with patterns like "901" in "901-333-".
The dollar ( $ ) symbolIt tells the computer that the match must occur at the end of the string or before \n at the end of the line or string.Example : -\d{3}$  will match with patterns like "-333" in "-901-333".
Example : -\d{3}$  will match with patterns like "-333" in "-901-333".
Character ClassesA character class matches any one of a set of characters. It is used to match the most basic element of a language like a letter, a digit, space, a symbol etc./s : matches any whitespace characters such as space and tab/S : matches any non-whitespace characters/d : matches any digit character/D : matches any non-digit characters/w : matches any word character (basically alpha-numeric)/W : matches any non-word character/b : matches any word boundary (this would include spaces, dashes, commas, semi-colons, etc)[set_of_characters] – Matches any single character in set_of_characters. By default, the match is case-sensitive.Example : [abc] will match characters a,b and c in any string.[^set_of_characters] – Negation: Matches any single character that is not in set_of_characters. By default, the match is case sensitive.Example : [^abc] will match any character except a,b,c .[first-last] – Character range: Matches any single character in the range from first to last.Example : [a-zA-z] will match any character from a to z or A to Z.
/s : matches any whitespace characters such as space and tab/S : matches any non-whitespace characters/d : matches any digit character/D : matches any non-digit characters/w : matches any word character (basically alpha-numeric)/W : matches any non-word character/b : matches any word boundary (this would include spaces, dashes, commas, semi-colons, etc)
[set_of_characters] – Matches any single character in set_of_characters. By default, the match is case-sensitive.
Example : [abc] will match characters a,b and c in any string.
[^set_of_characters] – Negation: Matches any single character that is not in set_of_characters. By default, the match is case sensitive.
Example : [^abc] will match any character except a,b,c .
[first-last] – Character range: Matches any single character in the range from first to last.
Example : [a-zA-z] will match any character from a to z or A to Z.
The Escape Symbol : \If you want to match for the actual ‘+’, ‘.’ etc characters, add a backslash( \ ) before that character. This will tell the computer to treat the following character as a search character and consider it for matching pattern.Example : \d+[\+-x\*]\d+ will match patterns like "2+2"
and "3*9" in "(2+2) * 3*9".
If you want to match for the actual ‘+’, ‘.’ etc characters, add a backslash( \ ) before that character. This will tell the computer to treat the following character as a search character and consider it for matching pattern.
Example : \d+[\+-x\*]\d+ will match patterns like "2+2"
and "3*9" in "(2+2) * 3*9".
Grouping Characters ( )A set of different symbols of a regular expression can be grouped together to act as a single unit and behave as a block, for this, you need to wrap the regular expression in the parenthesis( ).Example : ([A-Z]\w+) contains two different elements of the regular 
expression combined together. This expression will match any pattern 
containing uppercase letter followed by any character.
A set of different symbols of a regular expression can be grouped together to act as a single unit and behave as a block, for this, you need to wrap the regular expression in the parenthesis( ).
Example : ([A-Z]\w+) contains two different elements of the regular 
expression combined together. This expression will match any pattern 
containing uppercase letter followed by any character.
Vertical Bar ( | ) :Matches any one element separated by the vertical bar (|) character.Example :  th(e|is|at) will match words - the, this and that.
Vertical Bar ( | ) :Matches any one element separated by the vertical bar (|) character.
Example :  th(e|is|at) will match words - the, this and that.
\number :Backreference: allows a previously matched sub-expression(expression captured or enclosed within circular brackets ) to be identified subsequently in the same regular expression. \n means that group enclosed within the n-th bracket will be repeated at current position.Example : ([a-z])\1 will match “ee” in Geek because the character 
at second position is same as character at position 1 of the match.
Example : ([a-z])\1 will match “ee” in Geek because the character 
at second position is same as character at position 1 of the match.
Comment : (?# comment) –Inline comment: The comment ends at the first closing parenthesis.Example : \bA(?#This is an inline comment)\w+\b# [to end of line] : X-mode comment. The comment starts at an unescaped # and continues to the end of the line.Example :  (?x)\bA\w+\b#Matches words starting with A
Example : \bA(?#This is an inline comment)\w+\b
# [to end of line] : X-mode comment. The comment starts at an unescaped # and continues to the end of the line.
Example :  (?x)\bA\w+\b#Matches words starting with A
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