Python check string with regex

Python RegEx

A RegEx, or Regular Expression, is a sequence of characters that forms a search pattern.

RegEx can be used to check if a string contains the specified search pattern.

RegEx Module

Python has a built-in package called re , which can be used to work with Regular Expressions.

RegEx in Python

When you have imported the re module, you can start using regular expressions:

Example

Search the string to see if it starts with «The» and ends with «Spain»:

txt = «The rain in Spain»
x = re.search(«^The.*Spain$», txt)

RegEx Functions

The re module offers a set of functions that allows us to search a string for a match:

Function	Description
findall	Returns a list containing all matches
search	Returns a Match object if there is a match anywhere in the string
split	Returns a list where the string has been split at each match
sub	Replaces one or many matches with a string

Metacharacters

Metacharacters are characters with a special meaning:

Character	Description	Example	Try it
[]	A set of characters	«[a-m]»	Try it »
\	Signals a special sequence (can also be used to escape special characters)	«\d»	Try it »
.	Any character (except newline character)	«he..o»	Try it »
^	Starts with	«^hello»	Try it »
$	Ends with	«planet$»	Try it »
*	Zero or more occurrences	«he.*o»	Try it »
+	One or more occurrences	«he.+o»	Try it »
?	Zero or one occurrences	«he.?o»	Try it »
<>	Exactly the specified number of occurrences	«he.o»	Try it »
|	Either or	«falls|stays»	Try it »
()	Capture and group

Special Sequences

A special sequence is a \ followed by one of the characters in the list below, and has a special meaning:

Character	Description	Example	Try it
\A	Returns a match if the specified characters are at the beginning of the string	«\AThe»	Try it »
\b	Returns a match where the specified characters are at the beginning or at the end of a word (the «r» in the beginning is making sure that the string is being treated as a «raw string»)	r»\bain» r»ain\b»	Try it » Try it »
\B	Returns a match where the specified characters are present, but NOT at the beginning (or at the end) of a word (the «r» in the beginning is making sure that the string is being treated as a «raw string»)	r»\Bain» r»ain\B»	Try it » Try it »
\d	Returns a match where the string contains digits (numbers from 0-9)	«\d»	Try it »
\D	Returns a match where the string DOES NOT contain digits	«\D»	Try it »
\s	Returns a match where the string contains a white space character	«\s»	Try it »
\S	Returns a match where the string DOES NOT contain a white space character	«\S»	Try it »
\w	Returns a match where the string contains any word characters (characters from a to Z, digits from 0-9, and the underscore _ character)	«\w»	Try it »
\W	Returns a match where the string DOES NOT contain any word characters	«\W»	Try it »
\Z	Returns a match if the specified characters are at the end of the string	«Spain\Z»	Try it »

Sets

A set is a set of characters inside a pair of square brackets [] with a special meaning:

Set	Description	Try it
[arn]	Returns a match where one of the specified characters ( a , r , or n ) is present	Try it »
[a-n]	Returns a match for any lower case character, alphabetically between a and n	Try it »
[^arn]	Returns a match for any character EXCEPT a , r , and n	Try it »
[0123]	Returns a match where any of the specified digits ( 0 , 1 , 2 , or 3 ) are present	Try it »
3	Returns a match for any digit between 0 and 9	Try it »
48	Returns a match for any two-digit numbers from 00 and 59	Try it »
[a-zA-Z]	Returns a match for any character alphabetically between a and z , lower case OR upper case	Try it »
[+]	In sets, + , * , . , | , () , $ , <> has no special meaning, so [+] means: return a match for any + character in the string	Try it »

The findall() Function

The findall() function returns a list containing all matches.

Example

Print a list of all matches:

txt = «The rain in Spain»
x = re.findall(«ai», txt)
print(x)

The list contains the matches in the order they are found.

If no matches are found, an empty list is returned:

Example

Return an empty list if no match was found:

txt = «The rain in Spain»
x = re.findall(«Portugal», txt)
print(x)

The search() Function

The search() function searches the string for a match, and returns a Match object if there is a match.

If there is more than one match, only the first occurrence of the match will be returned:

Example

Search for the first white-space character in the string:

txt = «The rain in Spain»
x = re.search(«\s», txt)

print(«The first white-space character is located in position:», x.start())

If no matches are found, the value None is returned:

Example

Make a search that returns no match:

txt = «The rain in Spain»
x = re.search(«Portugal», txt)
print(x)

The split() Function

The split() function returns a list where the string has been split at each match:

Example

Split at each white-space character:

txt = «The rain in Spain»
x = re.split(«\s», txt)
print(x)

You can control the number of occurrences by specifying the maxsplit parameter:

Example

Split the string only at the first occurrence:

txt = «The rain in Spain»
x = re.split(«\s», txt, 1)
print(x)

The sub() Function

The sub() function replaces the matches with the text of your choice:

Example

Replace every white-space character with the number 9:

txt = «The rain in Spain»
x = re.sub(«\s», «9», txt)
print(x)

You can control the number of replacements by specifying the count parameter:

Example

Replace the first 2 occurrences:

txt = «The rain in Spain»
x = re.sub(«\s», «9», txt, 2)
print(x)

Match Object

A Match Object is an object containing information about the search and the result.

Note: If there is no match, the value None will be returned, instead of the Match Object.

Example

Do a search that will return a Match Object:

txt = «The rain in Spain»
x = re.search(«ai», txt)
print(x) #this will print an object

The Match object has properties and methods used to retrieve information about the search, and the result:

.span() returns a tuple containing the start-, and end positions of the match.
.string returns the string passed into the function
.group() returns the part of the string where there was a match

Example

Print the position (start- and end-position) of the first match occurrence.

The regular expression looks for any words that starts with an upper case «S»:

txt = «The rain in Spain»
x = re.search(r»\bS\w+», txt)
print(x.span())

Example

Print the string passed into the function:

txt = «The rain in Spain»
x = re.search(r»\bS\w+», txt)
print(x.string)

Example

Print the part of the string where there was a match.

The regular expression looks for any words that starts with an upper case «S»:

txt = «The rain in Spain»
x = re.search(r»\bS\w+», txt)
print(x.group())

Note: If there is no match, the value None will be returned, instead of the Match Object.

Источник

How to check a valid regex string using Python?

A Regex (Regular Expression) is a sequence of characters used for defining a pattern. This pattern could be used for searching, replacing and other operations. Regex is extensively utilized in applications that require input validation, Password validation, Pattern Recognition, search and replace utilities (found in word processors) etc. This is due to the fact that regex syntax stays the same across different programming languages and implementations. Therefore, one having the grasp of it provides longevity across languages. In this article, we will be creating a program for checking the validity of a regex string.

The method we would be using will require a firm understanding of the try-except construct of python. Therefore, it would be wise if we touch upon that before moving over to the actual code.

Exception handling

Try except block is used to catch and handle exceptions encountered during the execution of a particular block of code (construct exists in other programming languages under the name try-catch). The general syntax of a try-except block is as follows:

except [Exception]:

# Execute this code, if an exception arises during the execution of the try block

.

.

In the above syntax, any code found within the try block would be executed. If an exception/error arises during the execution of the try block then (only) the except block is executed. If the try block executes without producing an exception, then the except block won’t be executed. If a bare except clause is used, then it would catch any exception (and certain even System_Exits) encountered during the execution of try block. To prevent such from happening, it is generally a good practice to specify an exception after the except. This ensures that only after encountering that specific exception/error, the except block will execute. This prevents concealment of other errors encountered during the execution of the try block. Also, multiple except clauses can be used within the same try-except block, this enables it to trap a plethora of exceptions, and deal with them specifically. This construct contains other keywords as well such as finally, else etc. which aren’t required in current context. Therefore, only the relevant sections are described.

In the following code, we would be specifying re.error as the exception in the except clause of the try-except block. This error is encountered when an invalid regex pattern is found, during the compilation of the pattern.

Python3

Explanation:

Firstly we imported the re library, for enabling regex functionality in our code. Then we assigned a string containing the regex pattern to the variable pattern. The pattern provided is invalid as it contains an unclosed character class (in regex square brackets `[ ]`are used for defining a character class). We placed the re.compile() (used to compile regex patterns) function within the try block. This will firstly try to compile the pattern and if any exception occurs during the compilation, it would firstly check whether it is re.error, if it is then only the except block will execute. Otherwise, the exception will be displayed in the traceback, leading to program termination. The except block contains print statements that outputs the user defined message to the stdout and then exits the program (via exit()). Since the pattern provided is invalid (explained earlier) this lead to the except block getting executed.

The above code only deals with re.error exception. But other exceptions related to regex also exist such as RecursionError, Which needs to be dealt with specifically(by adding a separate except clause for that exception as well or changing the maximum stack depth using sys.setrecursionlimit() as of this case).

Checking whether the input string matches the Regex pattern

In the following example, we will test whether an input string matches a given regex pattern or not. This is assuming the regex pattern is a valid one (could be ensured using the aforementioned example). We would be checking whether the input string is a alphanumeric string (one containing alphabets or digits throughout its length) or not. We would be using the following class for checking the string:

Even though there exists a special sequence in regex (\w)for finding alphanumeric characters. But we won’t be using it as it contains the underscore ( _ ) in its character class (A-Za-z0-9_), which isn’t considered as an alphanumeric character under most standards.

Источник