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

Dec 05, 2019 · 5 mins read

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')

# \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)


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