Search the Community

The full form of regex is Regular Expression. It is an important feature of any programming language. It is a string pattern that is used to match, search, or replace the strings in a string value. The regex pattern can be used in the Python script using the “re” module of Python. This module has many types of functions to do different string operations. Different metacharacters and special sequences are used to define the regex patterns to search or replace the tasks. The purposes of using some commonly used metacharacters, special sequences, and regex methods in the Python script are shown in this tutorial. Some commonly used metacharacters in regex: Characters Purpose ‘+’ It is used to match one or more occurrences of a particular character in a string. ‘*’ It is used to match zero or more occurrences of a particular character in a string. ‘?’ It is used to match zero or one occurrence of a particular character in a string. ‘^’ It is used to match the particular character or string at the beginning of the string. ‘$’ It is used to match the particular character or string at the end of the string. ‘|’ It is used to match any of the multiple strings in a string. It works like the OR logic. ‘[]’ It is used to match a range of characters. ‘{}’ It is used to match a specific number of characters. Some commonly used special sequences in regex: Sequences Purpose ‘\A’ It is used to match the particular character at the start of the string. It works like the “^” character. ‘\b’, ‘\B’ The “\b” is used to match the string that contains the particular character or word at the beginning or end of the string. The “\B” works opposite to “\b”. ‘\d’, ‘\D’ The “\d” is used to match the decimal number in the string that is similar to “[0-9]”. The “\D” works opposite to “\d”. ‘\s’, ‘\S’ The “\s” is used to match the whitespace in the string that is similar to “[\n\t\r\v]”. The “\S” works opposite to “\s”. ‘\w’, ‘\W’ The “\w” is used to match the alphabetic and numeric characters in the string. The “\W” works opposite to “\w”. ‘\Z’ It is used to match the particular character at the end of the string. It works like the “$” character. Example 1: Match the String Using the Match() Function The match() function is used to match a regex pattern at the beginning of the string. The syntax of this function is given as follows: Syntax: re.match(pattern, string, flags=0) Here, the first argument is used to define the regex pattern. The second argument is used to define the main string. The third argument is optional and is used to define different types of flags. Create a Python file with the following script that matches a regex pattern with a defined string using the match() function. First, a defined regex pattern is used to match. Next, a search word is taken from the user and is used as a regex pattern to match with the string value. If any match is found, the search word is printed. Otherwise, the “No matching value found” string is printed. #Import necessary module import re #Define the function to print the matching result def matchString(): #Check the return value of the match() function if mat != None: print ("'" + mat.group() + "' is found in '" + strValue + "'") else: print ("No matching value found.") #Define the string value strValue = "First in first out." #Match the string based on the pattern mat = re.match('^First', strValue) #Call function to print the match result matchString () #Take the search string inValue = input("Enter the search value: ") mat = re.match(inValue + , strValue) #Call function to print the match result matchString () The following output appears for the “first” input value: Example 2: Find the String Using the Findall() Function The findall() function is used to return all matching words that are found in the main string as a tuple. Syntax: re.findall(pattern, string, flags=0) Here, the first argument is used to define the regex pattern. The second argument is used to define the main string. The third argument is optional and is used to define different types of flags. Create a Python file with the following script that takes a main string value and a search string value from the user. Next, use the search word in the regex pattern to find the search word in the main string. The number of total matches are printed in the output. #Import necessary module import re #Take a string value inValue = input("Enter a string: ") #Take a search word srcValue = input("Enter a search word: ") #Search the word in the string srcResult = re.findall(srcValue + "\w*", inValue) #Print the search result print ("The word '" + srcValue + "' is found in the string " + str(len(srcResult)) + " times.") According to the output, the search word “eat” is found twice in the “We eat to live and don’t live to eat” main string. Example 3: Search the String Using the Search() Function The search() is another function to search a particular pattern in a string value. It contains the same arguments as the match() and findall() functions. Create a Python file with the following script that searches the word “Python” in a string value that will be taken from the user. If the search word exists in the input value, a success message is printed. Otherwise, a failure message is printed. #Import re module import re #Take a string value inValue = input("Enter a string: ") #Search the particular word in the string value srcResult = re.search( r'Python\w*', inValue) #Check the search word is found or not if srcResult: print ("'" + srcResult.group() + "' is found in '" + inValue + "'") else: print ("The search string is not found.") Output: The following output appears if the input string is “I like Python programming”: The following output appears if the input string is “I like PHP programming”: Example 4: Replace the String Using the Sub() Function The sub() function is used to search a particular string based on the pattern and replace it with another word. The syntax of this function is given as follows: Syntax: re.sub(pattern, replace_string, main_string) The first argument of this function contains the pattern that is used to search the particular string in the main string. The second argument of this function contains the “replace” string value. The third argument of this function contains the main string. This function returns the replaced string if any matching word exists in the main string based on the first argument. Create a Python file with the following script that searches for two digits at the end of the string. If the string contains two digits at the end, the digits are replaced by the “$50” string. #Import re module import re #Define the main string strValue = "The book price is 70" #Define the search pattern pattern = '[0-9]{2}' #Define the replace value replaceValue = '$50' #Search and replace the string based on the pattern modified_strValue = re.sub(pattern, replaceValue, strValue) #Print the original and modified string values print ("Original string: " + strValue) print ("Modified string: " + modified_strValue) Output: There were 70 at the end of the main string. So, the 70 is replaced by $50 in the replaced string. Example 5: Replace the String Using the Subn() Function The subn() function works like the sub() function, except it returns the output as a tuple where the first index contains the replaced value and the second index contains the total number of matches. Create a Python file with the following script that searches the alphabets A to L in the “LinuxHint.com” string using the subn() function: #Import re module import re #Define the main string strValue = "LinuxHint.com" #Define the search pattern pattern = '[A-L]' #Define the replace value replaceValue = '*' #Search and replace the string based on the pattern modified_strValue = re.subn(pattern, replaceValue, strValue) #Print the original string and the output of the subn() print ("Original string:\n" + strValue) print ("Output of subn() function: ") print (modified_strValue) Output: According to the following output, the “L” and “H” characters are replaced by the “*” character. Example 6: Split the String Using the Split() Function Create a Python file with the following script that used the split() function to divide the main string into multiple parts based on the regex pattern: #Import re module import re #Define string value strVal = "Rupa Akter;Nira Chowdhury;Mazharul Islam" #Define the pattern that will be used to split the data pattern = '[^A-Za-z ]' #Store the split values in a list split_result = re.split(pattern, strVal) print ("Output of the split() function:") print (split_result) Output: According to the output, the main string is divided into three parts based on the “[^A-Za-z ]” pattern that is used in the script. Conclusion The purpose of the most commonly used metacharacters, ranges, and Python built-in functions to search, replace, and split the strings are shown in this tutorial using simple Python scripts. View the full article

AWS WAF has extended support of the ‘ruleMatchDetails’ field in the logs to include Regex rules. Customers use logs to further investigate the requests that are inspected by AWS WAF. The ‘ruleMatchDetails’ log field gives visibility into the part of a request that matched a rule or rule group. View the full article

Regular expressions (regex) are a powerful tool for defining patterns within text. These patterns serve as robust mechanisms for searching, matching, and manipulating text, significantly reducing the amount of code and effort required to perform complex text-processing tasks. When used within Bash scripts, regex can help automate and streamline a variety of operations. In this blog post, we'll briefly touch upon what regex is, explore some common regex metacharacters, and demonstrate how to use regex inside Bash scripts. Whether you’re a system admin, a programmer, or someone curious about improving your scripting skills, knowing how to use regex in shell scripts is a valuable skill. So, without further ado, let’s dive in! ... View the full article