Reg Ex

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 

• 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.

• 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.
  

• Wildcard – ( . ) 
  The dot symbol can take place of any other symbol, that is why it
  is called the wildcard character.

  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 may
  or 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.

• 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-".

• The dollar ( $ ) symbol
  It 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".

• Character Classes
  A 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)

  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.[set_of_characters] – Matches any single character in set_of_characters. By default, the match is case-sensitive.

• 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".

• 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.

• 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.

• 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

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