python, regularexpression,

How to use regular expressions and extract all email addresses from a text file.

Follow Dec 05, 2019 · 5 mins read
How to use regular expressions and extract all email addresses from a text file.
Share this

Basics of Regular expression

**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)

  • [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 “39” in “(2+2) * 39”.

  • 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

Code

# Python program to extract emails From text
# the String By Regular Expression. 
  
# Importing module required for regular 
# expressions 
import re  

s =  open("test.txt",'r',encoding = 'utf-8')


contents = s.read()

# \S matches any non-whitespace character 
# @ for as in the Email 
# + for Repeats a character one or more times 
lst = re.findall('[^,;\s][email protected][^,;\s]+', contents)     
  
# Printing of List 
print(lst) 

Thank you Share if you like the post……. #python #regularexpression #howto #programming

Join Newsletter
Get the latest news right in your inbox. We never spam!
Written by