# Tag: regular expression

## Regular Expressions

Regular expressions are essentially a highly specialized programming language embedded inside Python that empowers you to specify the rules for the set of possible strings that you want to match.

In Python you need the re module for regular expressions usage. The grammar overview is on the bottom of this page.

Related course:
Complete Python Bootcamp: Go from zero to hero in Python

## The Match function

The match function is defined as:

The parameters are:

Parameters Description
pattern a regular expression
string the input string

If you want to match a string to a numberic sequence of exactly five, you can use this code:

Example outputs:

## Email validation regex

We can use the same function to validate email address. The grammar rules are seen in re.compile and in the grammar table.

String Match
12345 True
12358 True
55555 True
123 False
123K5 False
5555555 False
 #!/usr/bin/pythonimport reinput = raw_input("Enter an input string:")m = re.match('[^@][email protected][^@]+\.[^@]+',input)if m: print("True")else: print("False")

## The Search Function

The search function is defined as:

The parameters are:

Parameter Description
pattern a regular expression, defines the string to be searched
string the search space

To search if an e-mail address is in a string:

## Regular Expression Examples

A few examples of regular expressions:

## Regular Expression Grammar

Overview of the regex grammar:
Example Regex
Email [^@][email protected][^@]+\.[^@]+
Date MM/DD/YY (\d+/\d+/\d+)
Integer (positive) (?<![-.])\b[0-9]+\b(?!\.[0-9])
Integer [+-]?(?<!\.)\b[0-9]+\b(?!\.[0-9])
Float (?<=>)\d+.\d+|\d+

Regex Description
\d Matches any decimal digit; this is equivalent to the class [0-9]
\D Matches any non-digit character; this is equivalent to the class [^0-9].
\s Matches any whitespace character; this is equivalent to the class [ \t\n\r\f\v].
\S Matches any non-whitespace character; this is equivalent to the class [^ \t\n\r\f\v].
\w Matches any alphanumeric character; this is equivalent to the class [a-zA-Z0-9_].
\W Matches any non-alphanumeric character; this is equivalent to the class [^a-zA-Z0-9_].
\Z Matches only at end of string
[..] Match single character in brackets
[^..] Match any single character not in brackets
. Match any character except newline
\$ Match the end of the string
* Match 0 or more repetitions
+ 1 or more repetitions
{m} Exactly m copies of the previous RE should be matched.
| Match A or B. A|B
? 0 or 1 repetitions of the preceding RE
[a-z] Any lowercase character
[A-Z] Any uppercase character
[a-zA-Z] Any character
[0-9] Any digit