PythonRegEx
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 calledre, which can be used to work with Regular Expressions.
Import there module:
RegEx in Python
When you have imported there 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
There 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 aMatch 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.{2}o" | Try it » |
| | | Either or | "falls|stays" | Try it » |
| () | Capture and group |
Flags
You can add flags to the pattern when using regular expressions.
| Flag | Shorthand | Description | Try it |
|---|---|---|---|
| re.ASCII | re.A | Returns only ASCII matches | Try it » |
| re.DEBUG | Returns debug information | Try it » | |
| re.DOTALL | re.S | Makes the . character match all characters (including newline character) | Try it » |
| re.IGNORECASE | re.I | Case-insensitive matching | Try it » |
| re.MULTILINE | re.M | Returns only matches at the beginning of each line | Try it » |
| re.NOFLAG | Specifies that no flag is set for this pattern | ||
| re.UNICODE | re.U | Returns Unicode matches. This is default from Python 3. For Python 2: use this flag to return only Unicode matches | Try it » |
| re.VERBOSE | re.X | Allows whitespaces and comments inside patterns. Makes the pattern more readable | Try it » |
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, orn) is present | Try it » |
| [a-n] | Returns a match for any lower case character, alphabetically betweena andn | Try it » |
| [^arn] | Returns a match for any character EXCEPTa,r, andn | Try it » |
| [0123] | Returns a match where any of the specified digits (0,1,2, or3) are present | Try it » |
| [0-9] | Returns a match for any digit between0 and9 | Try it » |
| [0-5][0-9] | Returns a match for any two-digit numbers from00 and59 | Try it » |
| [a-zA-Z] | Returns a match for any character alphabetically betweena andz, 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
Thefindall() 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
Thesearch() function searches the string for a match, and returns aMatch 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 valueNone 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
Thesplit() 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 themaxsplit 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
Thesub() 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 thecount 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 valueNone 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 valueNone will be returned, instead of the Match Object.
