What is Regular Expression in Python?
ARegular Expression (RE)in a programming language is a special text string used for describing a search pattern. It is extremely useful for extracting information from text such as code, files, log, spreadsheets or even documents.
While using the Python regular expression the first thing is to recognize is that everything is essentially a character, and we are writing patterns to match a specific sequence of characters also referred as string. Ascii or latin letters are those that are on your keyboards and Unicode is used to match the foreign text. It includes digits and punctuation and all special characters like $#@!%, etc.
In this Python RegEx tutorial, we will learn-
- Regular Expression Syntax
- Example of w+ and ^ Expression
- Example of \s expression in re.split function
- Using regular expression methods
- Using re.match()
- Finding Pattern in Text (re.search())
- Using re.findall for text
- Python Flags
- Example of re.M or Multiline Flags
For instance, a Python regular expression could tell a program to search for specific text from the string and then to print out the result accordingly. Expression can include
- Text matching
- Repetition
- Branching
- Pattern-composition etc.
Regular expression or RegEx in Python is denoted as RE (REs, regexes or regex pattern) are imported throughre module. Python supports regular expression through libraries. RegEx in Python supports various things likeModifiers, Identifiers, and White space characters.
| Identifiers | Modifiers | White space characters | Escape required |
|---|---|---|---|
| \d= any number (a digit) | \d represents a digit.Ex: \d{1,5} it will declare digit between 1,5 like 424,444,545 etc. | \n = new line | . + * ? [] $ ^ () {} | \ |
| \D= anything but a number (a non-digit) | + = matches 1 or more | \s= space | |
| \s = space (tab,space,newline etc.) | ? = matches 0 or 1 | \t =tab | |
| \S= anything but a space | * = 0 or more | \e = escape | |
| \w = letters ( Match alphanumeric character, including “_”) | $ match end of a string | \r = carriage return | |
| \W =anything but letters ( Matches a non-alphanumeric character excluding “_”) | ^ match start of a string | \f= form feed | |
| . = anything but letters (periods) | | matches either or x/y | —————– | |
| \b = any character except for new line | [] = range or “variance” | —————- | |
| \. | {x} = this amount of preceding code | —————– |
Regular Expression(RE) Syntax
import re
- “re” module included with Python primarily used for string searching and manipulation
- Also used frequently for web page “Scraping” (extract large amount of data from websites)
We will begin the expression tutorial with this simple exercise by using the expressions (w+) and (^).
Example of w+ and ^ Expression
- “^”:This expression matches the start of a string
- “w+“: This expression matches the alphanumeric character in the string
Here we will see a Python RegEx Example of how we can use w+ and ^ expression in our code. We cover the function re.findall() in Python, later in this tutorial but for a while we simply focus on \w+ and \^ expression.
For example, for our string “guru99, education is fun” if we execute the code with w+ and^, it will give the output “guru99”.
import rexx = "guru99,education is fun"r1 = re.findall(r"^\w+",xx)print(r1)
Remember, if you remove +sign from the w+, the output will change, and it will only give the first character of the first letter, i.e., [g]
Example of \s expression in re.split function
- “s”: This expression is used for creating a space in the string
To understand how this RegEx in Python works, we begin with a simple Python RegEx Example of a split function. In the example, we have split each word using the “re.split” function and at the same time we have used expression \s that allows to parse each word in the string separately.
When you execute this code it will give you the output [‘we’, ‘are’, ‘splitting’, ‘the’, ‘words’].
Now, let see what happens if you remove “\” from s. There is no ‘s’ alphabet in the output, this is because we have removed ‘\’ from the string, and it evaluates “s” as a regular character and thus split the words wherever it finds “s” in the string.
Similarly, there are series of other Python regular expression that you can use in various ways in Python like \d,\D,$,\.,\b, etc.
Here is the complete code
import rexx = "guru99,education is fun"r1 = re.findall(r"^\w+", xx)print((re.split(r'\s','we are splitting the words')))print((re.split(r's','split the words')))
Next, we will going to see the types of methods that are used with regular expression in Python.
Using regular expression methods
The “re” package provides several methods to actually perform queries on an input string. We will see the methods of re in Python:
- re.match()
- re.search()
- re.findall()
Note: Based on the regular expressions, Python offers two different primitive operations. The match method checks for a match only at the beginning of the string while search checks for a match anywhere in the string.
re.match()
re.match()function of re in Python will search the regular expression pattern and return the first occurrence. The Python RegEx Match method checks for a match only at the beginning of the string. So, if a match is found in the first line, it returns the match object. But if a match is found in some other line, the Python RegEx Match function returns null.
For example, consider the following code of Python re.match() function. The expression “w+” and “\W” will match the words starting with letter ‘g’ and thereafter, anything which is not started with ‘g’ is not identified. To check match for each element in the list or string, we run the forloop in this Python re.match() Example.
re.search(): Finding Pattern in Text
re.search()function will search the regular expression pattern and return the first occurrence. Unlike Python re.match(), it will check all lines of the input string. The Python re.search() function returns a match object when the pattern is found and “null” if the pattern is not found
How to use search()?
In order to use search() function, you need to import Python re module first and then execute the code. The Python re.search() function takes the “pattern” and “text” to scan from our main string
For example here we look for two literal strings “Software testing” “guru99”, in a text string “SoftwareTestingis fun”. For “software testing” we found the match hence it returns the output of Python re.search() Example as “found a match”, while for word “guru99” we could not found in string hence it returns the output as “No match”.
re.findall()
findall()module is used to search for “all” occurrences that match a given pattern. In contrast, search() module will only return the first occurrence that matches the specified pattern. findall() will iterate over all the lines of the file and will return all non-overlapping matches of pattern in a single step.
How to Use re.findall() in Python?
Here we have a list of e-mail addresses, and we want all the e-mail addresses to be fetched out from the list, we use the method re.findall() in Python. It will find all the e-mail addresses from the list.
Here is the complete code for Example of re.findall()
import relist = ["guru99 get", "guru99 give", "guru Selenium"]for element in list: z = re.match("(g\w+)\W(g\w+)", element)if z: print((z.groups())) patterns = ['software testing', 'guru99']text = 'software testing is fun?'for pattern in patterns: print('Looking for "%s" in "%s" ->' % (pattern, text), end=' ') if re.search(pattern, text): print('found a match!')else: print('no match')abc = 'guru99@google.com, careerguru99@hotmail.com, users@yahoomail.com'emails = re.findall(r'[\w\.-]+@[\w\.-]+', abc)for email in emails: print(email)Python Flags
Many Python Regex Methods and Regex functions take an optional argument called Flags. This flags can modify the meaning of the given Python Regex pattern. To understand these we will see one or two example of these Flags.
Various flags used in Python includes
| Syntax for Regex Flags | What does this flag do |
|---|---|
| [re.M] | Make begin/end consider each line |
| [re.I] | It ignores case |
| [re.S] | Make [ . ] |
| [re.U] | Make { \w,\W,\b,\B} follows Unicode rules |
| [re.L] | Make {\w,\W,\b,\B} follow locale |
| [re.X] | Allow comment in Regex |
Example of re.M or Multiline Flags
In multiline the pattern character [^] match the first character of the string and the beginning of each line (following immediately after the each newline). While expression small “w” is used to mark the space with characters. When you run the code the first variable “k1” only prints out the character ‘g’ for word guru99, while when you add multiline flag, it fetches out first characters of all the elements in the string.
Here is the code
import rexx = """guru99 careerguru99selenium"""k1 = re.findall(r"^\w", xx)k2 = re.findall(r"^\w", xx, re.MULTILINE)print(k1)print(k2)
- We declared the variable xx for string ” guru99…. careerguru99….selenium”
- Run the code without using flags multiline, it gives the output only ‘g’ from the lines
- Run the code with flag “multiline”, when you print ‘k2’ it gives the output as ‘g’, ‘c’ and ‘s’
- So, the difference we can see after and before adding multi-lines in above example.
Likewise, you can also use other Python flags like re.U (Unicode), re.L (Follow locale), re.X (Allow Comment), etc.
Python 2 Example
Above codes are Python 3 examples, If you want to run in Python 2 please consider following code.
# Example of w+ and ^ Expressionimport rexx = "guru99,education is fun"r1 = re.findall(r"^\w+",xx)print r1# Example of \s expression in re.split functionimport rexx = "guru99,education is fun"r1 = re.findall(r"^\w+", xx)print (re.split(r'\s','we are splitting the words'))print (re.split(r's','split the words'))# Using re.findall for textimport relist = ["guru99 get", "guru99 give", "guru Selenium"]for element in list: z = re.match("(g\w+)\W(g\w+)", element)if z: print(z.groups()) patterns = ['software testing', 'guru99']text = 'software testing is fun?'for pattern in patterns: print 'Looking for "%s" in "%s" ->' % (pattern, text), if re.search(pattern, text): print 'found a match!'else: print 'no match'abc = 'guru99@google.com, careerguru99@hotmail.com, users@yahoomail.com'emails = re.findall(r'[\w\.-]+@[\w\.-]+', abc)for email in emails: print email# Example of re.M or Multiline Flagsimport rexx = """guru99 careerguru99selenium"""k1 = re.findall(r"^\w", xx)k2 = re.findall(r"^\w", xx, re.MULTILINE)print k1print k2Summary
A regular expression in aprogramming languageis a special text string used for describing a search pattern. It includes digits and punctuation and all special characters like $#@!%, etc. Expression can include literal
- Text matching
- Repetition
- Branching
- Pattern-composition etc.
In Python, a regular expression is denoted as RE (REs, regexes or regex pattern) are embedded through Python re module.
- “re” module included with Python primarily used for string searching and manipulation
- Also used frequently for webpage “Scraping” (extract large amount of data from websites)
- Regular Expression Methods includere.match(),re.search()& re.findall()
- Other Python RegEx replace methods are sub() and subn() which are used to replace matching strings in re
- Python FlagsMany Python Regex Methods and Regex functions take an optional argument called Flags
- This flags can modify the meaning of the given Regex pattern
- Various Python flags used in Regex Methods are re.M, re.I, re.S, etc.
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FAQs
What is search () match () and Findall () in regular expression? ›
findall() module is used to search for “all” occurrences that match a given pattern. In contrast, search() module will only return the first occurrence that matches the specified pattern. findall() will iterate over all the lines of the file and will return all non-overlapping matches of pattern in a single step.
Is there any difference between re match () and re search () in the Python re module? ›re.search searches for the pattern throughout the string, whereas re. match does not search the pattern; if it does not, it has no other choice than to match it at start of the string.
What is the difference between re search and re Findall in Python? ›Above we used re.search() to find the first match for a pattern. findall() finds *all* the matches and returns them as a list of strings, with each string representing one match.
What is difference between match () and search () function? ›Python offers two different primitive operations based on regular expressions: match checks for a match only at the beginning of the string, while search checks for a match anywhere in the string (this is what Perl does by default).
How do I use regex to match? ›To match a character having special meaning in regex, you need to use a escape sequence prefix with a backslash ( \ ). E.g., \. matches "." ; regex \+ matches "+" ; and regex \( matches "(" . You also need to use regex \\ to match "\" (back-slash).
Which regex will match everything? ›Throw in an * (asterisk), and it will match everything. Read more. \s (whitespace metacharacter) will match any whitespace character (space; tab; line break; ...), and \S (opposite of \s ) will match anything that is not a whitespace character.
How do you search and replace with regex in Python? ›To replace a string in Python, the regex sub() method is used. It is a built-in Python method in re module that returns replaced string. Don't forget to import the re module. This method searches the pattern in the string and then replace it with a new given expression.
How do you get multiple matches in regex Python? ›To access multiple matches of a regex group in Python, you can use the re. finditer() or the re. findall() method.
What is the difference between regex match and regex search in Python? ›Matching Versus Searching
Python offers two different primitive operations based on regular expressions: match checks for a match only at the beginning of the string, while search checks for a match anywhere in the string (this is what Perl does by default).
A string within square brackets specifies any of the characters in string. Thus [abc], if compared to other strings, would match any that contained a, b, or c. Integer values enclosed in {} indicate the number of times to apply the preceding regular expression.
How do you search and replace with regex VS code? ›
- Ctrl+Shift+F or ⇧⌘F Search in files.
- Ctrl+Shift+H or ⇧⌘H Replace in files.
- Press Ctrl+R to open the search and replace pane. ...
- Enter a search string in the top field and a replace string in the bottom field. ...
- When you search for a text string that contains special regex symbols, GoLand automatically escapes them with backlash \ in the search field.
regex match() checks the pattern only at the beginning of the string, while regex search() checks the pattern anywhere in the string. The match() function returns the match object if a pattern is found, otherwise none. match() – Finds the pattern only at the beginning of the string and returns the matched object.
How does match () work? ›The MATCH function searches for a specified item in a range of cells, and then returns the relative position of that item in the range. For example, if the range A1:A3 contains the values 5, 25, and 38, then the formula =MATCH(25,A1:A3,0) returns the number 2, because 25 is the second item in the range.
What is the difference between match and test in regex? ›The test() method executes a search for a match between a regular expression and a specified string. Returns true or false. The match() method retrieves the matches when matching a string against a regular expression.
How to match two strings using regex? ›- Line 1: We create a string variable.
- Line 2: We create a regex with a bird pattern. The input is in lowercase. ...
- Line 5: We create a variable result. ...
- Line 6: The result variable will be printed to the console.
- Line 9: We create a variable result. ...
- Line 10: A Boolean value will be printed to the console.
Syntax: How to Match a String to a Regular Expression
Is the character string to match. For example, the regular expression '^Ste(v|ph)en$' matches values starting with Ste followed by either ph or v, and ending with en. Note: The output value is numeric.
If you need to know if a string matches a regular expression RegExp , use RegExp.prototype.test() . If you only want the first match found, you might want to use RegExp.prototype.exec() instead.
What is the most basic regex? ›The character class is the most basic regex concept after a literal match. It makes one small sequence of characters match a larger set of characters. For example, [A-Z] could stand for any uppercase letter in the English alphabet, and \d could mean any digit.
How do you match a specific set of characters in regex? ›With a “character class”, also called “character set”, you can tell the regex engine to match only one out of several characters. Simply place the characters you want to match between square brackets. If you want to match an a or an e, use [ae]. You could use this in gr[ae]y to match either gray or grey.
How do you find the exact match in regex Python? ›
- Using re.search() The re.search method searches the given string for a match to the specified regular expression pattern. ...
- Using re. match() ...
- Using re. fullmatch() ...
- Using re. findall() ...
- Using Pandas Series.str.extract()
match() function. When provided with a regular expression, the re. match() function checks the string to be matched for a pattern in the RegEx and returns the first occurrence of such a pattern match. This function only checks for a match at the beginning of the string.
How regex works in Python? ›Regular Expressions, also known as “regex” or “regexp”, are used to match strings of text such as particular characters, words, or patterns of characters. It means that we can match and extract any string pattern from the text with the help of regular expressions.
How do you replace all occurrences in string regex Python? ›sub() method will replace all pattern occurrences in the target string. By setting the count=1 inside a re. sub() we can replace only the first occurrence of a pattern in the target string with another string. Set the count value to the number of replacements you want to perform.
How to find all matches with groups in regex python? ›To capture all matches to a regex group we need to use the finditer() method. The finditer() method finds all matches and returns an iterator yielding match objects matching the regex pattern.
How to find all possible combinations of a string in Python? ›To find all possible permutations of a given string, you can use the itertools module which has a useful method called permutations(iterable[, r]). This method return successive r length permutations of elements in the iterable as tuples.
Which regex matches one or more times? ›The character + in a regular expression means "match the preceding character one or more times". For example A+ matches one or more of character A. The plus character, used in a regular expression, is called a Kleene plus .
Is regex faster than string matching? ›As the regex gets more complicated, it will take much more effort and complexity to write equivlent string manipulation code that performs well. Save this answer. Show activity on this post. String operations will always be faster than regular expression operations.
How do you test if two regex are the same? ›We say that two regular expressions R and S are equivalent if they describe the same language. In other words, if L(R) = L(S) for two regular expressions R and S then R = S.
What are the four typical uses of regex? ›- Input Validation.
- String Parsing.
- Syntax Highlighting.
- Data Scraping.
- Search.
- String manipulation.
- Data Mapping.
What are the two types of regex? ›
- DFA (POSIX or not—similar either way)
- Traditional NFA (most common: Perl, . NET, PHP, Java, Python, . . . )
- POSIX NFA.
A simple example for a regular expression is a (literal) string. For example, the Hello World regex matches the "Hello World" string. . (dot) is another example for a regular expression. A dot matches any single character; it would match, for example, "a" or "1".
Can I use replace () in regex? ›The Regex. Replace(String, String, MatchEvaluator, RegexOptions) method is useful for replacing a regular expression match if any of the following conditions is true: If the replacement string cannot readily be specified by a regular expression replacement pattern.
How to use regex to replace special characters in string? ›If you are having a string with special characters and want's to remove/replace them then you can use regex for that. Use this code: Regex. Replace(your String, @"[^0-9a-zA-Z]+", "")
How do I search and replace codes? ›You can use the Find control in code or text windows, such as Output windows and Find Results windows, by selecting Edit > Find and Replace or pressing Ctrl+F.
How do I find a specific letter in regex? ›- Use square brackets [] to match any characters in a set.
- Use \w to match any single alphanumeric character: 0-9 , a-z , A-Z , and _ (underscore).
- Use \d to match any single digit.
- Use \s to match any single whitespace character.
[A-Za-z] will match all the alphabets (both lowercase and uppercase). ^ and $ will make sure that nothing but these alphabets will be matched.
What is the difference between both the output search and Findall? ›Output. Here you can see that, search() method is able to find a pattern from any position of the string. The re. findall() helps to get a list of all matching patterns.
What is the difference between search and match in RegEx? ›Matching Versus Searching
Python offers two different primitive operations based on regular expressions: match checks for a match only at the beginning of the string, while search checks for a match anywhere in the string (this is what Perl does by default).
Match(String) Searches the specified input string for the first occurrence of the regular expression specified in the Regex constructor. Match(String, Int32) Searches the input string for the first occurrence of a regular expression, beginning at the specified starting position in the string.
What is the difference between match and test in RegEx? ›
The test() method executes a search for a match between a regular expression and a specified string. Returns true or false. The match() method retrieves the matches when matching a string against a regular expression.
What is the match function in RegEx in Python? ›match() function. When provided with a regular expression, the re. match() function checks the string to be matched for a pattern in the RegEx and returns the first occurrence of such a pattern match. This function only checks for a match at the beginning of the string.
What is the match function in RegEx Python? ›match() function finds and delivers the very first appearance of a regular expression pattern. In Python, the RegEx Match function solely searches for a matching string at the beginning of the provided text to be searched. The matching object is produced if one match is found in the first line.
How do you use the re search function in Python? ›Python Regex Search using re.search() Python regex re.search() method looks for occurrences of the regex pattern inside the entire target string and returns the corresponding Match Object instance where the match found. The re.search() returns only the first match to the pattern from the target string.
How do you match everything but character in regex? ›To match any character except a list of excluded characters, put the excluded charaters between [^ and ] . The caret ^ must immediately follow the [ or else it stands for just itself. The character '. ' (period) is a metacharacter (it sometimes has a special meaning).
What numbers do regex match? ›With regex you have a couple of options to match a digit. You can use a number from 0 to 9 to match a single choice. Or you can match a range of digits with a character group e.g. [4-9]. If the character group allows any digit (i.e. [0-9]), it can be replaced with a shorthand (\d).