Because regular expressions are so powerful, they are a useful way of switching on and off certain features (for example, whether to ignore capitalization when matching your regex). What’s the purpose of the flags argument?įlags allow you to control the regular expression engine. How to Use the Optional Flag Argument?Īs you’ve seen in the specification, the re.sub() method comes with an optional fourth flagargument: re.sub(pattern, repl, string, count=0, flags=0) You can also use positional arguments to save some characters: > re.sub('x+', '', s, 3)īut as many coders don’t know about the countargument, you probably should use the keyword argument for readability. In the first substitution operation, you replace only two occurrences of the pattern 'x+'. What if you don’t want to substitute all occurrences of a pattern but only a limited number of them? Just use the countargument! Here’s an example: > import re How to Use the Count Argument of the Regex Sub Method? Else it matches the shorter form 'sing', so it returns the shorter replacement string 'program' instead. If it matches the longer form 'singing', it returns 'programming'. The function takes the matched object as an input and returns a string. In this example, you first define a substitution function sub. # Learn to program because programming is fun. Text = 'Learn to sing because singing is fun.' You see where this leads us: the sub argument must be a function! So let’s try this: import re In this case, we need to substitute 'sing' with 'program' in some cases and 'sing' with 'programm' in other cases. You get the point: we don’t sing, we program.īut what if you want to actually fix this grammar mistake? After all, it’s programming, not programing. 'Learn to program because programing is fun.' > text = 'Learn to sing because singing is fun.' The easiest use is with only three arguments: the pattern 'sing‘, the replacement string 'program', and the string you want to modify ( text in our example). Let’s study some more examples-from simple to more complex. You’ll learn about those arguments in more detail later.Ī new string where count occurrences of the first substrings that match the pattern are replaced with the string value defined in the repl argument. The remaining two arguments are optional. The initial three arguments are required. Want to know how to use those flags? Check out this detailed article on the Finxter blog. flags(optional argument): a more advanced modifier that allows you to customize the behavior of the method.Per default, you use count=0 which reads as replace all occurrences of the pattern. count (optional argument): the maximum number of replacements you want to perform.The return value of the replacement function is a string that replaces the matching substring. If it’s a function, it needs to take one argument (the match object) which is passed for each occurrence of the pattern. repl: the replacement string or function.pattern: the regular expression pattern to search for strings you want to replace.The method has four arguments-two of which are optional. Specification re.sub(pattern, repl, string, count=0, flags=0) You can also see that the sub() method replaces all matched patterns in the string-not only the first one.īut there’s more! Let’s have a look at the formal definition of the sub() method. Note that you must escape the '+' symbol in 'C++' as otherwise it would mean the at-least-one regex. Your goal is to replace all those with the new string 'Python' (Python is the best language after all). You use the re.sub() method to search all of those occurrences. The text contains two occurrences of the string 'C++'. The re.sub(pattern, repl, string, count=0, flags=0) method returns a new string where all occurrences of the pattern in the old string are replaced by repl. Let’s answer the following question: How Does re.sub() Work in Python? Related article: Python Regex Superpower – The Ultimate Guideĭo you want to master the regex superpower? Check out my new book The Smartest Way to Learn Regular Expressions in Python with the innovative 3-step approach for active learning: (1) study a book chapter, (2) solve a code puzzle, and (3) watch an educational chapter video.
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