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.
Import the re module:
import re
RegEx in Python
When you have imported the re module, you can start using regular expressions:
例子
Search the string to see if it starts with "The" and ends with "Spain":
import re
txt = "The rain in Spain"
x = re.search("^The.*Spain$", txt)
亲自尝试 »
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 | 例子 | 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 |
Special Sequences
A special sequence is a \ followed by one of the characters in the list below, and has a special meaning:
| Character | Description | 例子 | 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 whitespace character | "\s" | Try it » |
| \S | Returns a match where the string DOES NOT contain a whitespace 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， 和 n |
Try it » |
| [0123] | Returns a match where any of the specified digits (0, 1, 2, or 3) are present |
Try it » |
| [0-9] | Returns a match for any digit between
0 and 9 |
Try it » |
| [0-5][0-9] | 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.
例子
Print a list of all matches:
import re
txt = "The rain in Spain"
x = re.findall("ai", txt)
print(x)
亲自尝试 »
The list contains the matches in the order they are found.
If no matches are found, an empty list is returned:
例子
Return an empty list if no match was found:
import re
txt = "The rain in Spain"
x = re.findall("Portugal", txt)
print(x)
亲自尝试 »
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:
例子
Search for the first white-space character in the string:
import re
txt = "The rain in Spain"
x = re.search("\s", txt)
print("The first white-space character is located in position:", x.start())
亲自尝试 »
If no matches are found, the value None is returned:
例子
Make a search that returns no match:
import re
txt = "The rain in Spain"
x = re.search("Portugal", txt)
print(x)
亲自尝试 »
The split() Function
The split() function returns a list where the string has been split at each match:
例子
Split at each white-space character:
import re
txt = "The rain in Spain"
x = re.split("\s", txt)
print(x)
亲自尝试 »
You can control the number of occurrences by specifying the maxsplit parameter:
例子
Split the string only at the first occurrence:
import re
txt = "The rain in Spain"
x = re.split("\s", txt, 1)
print(x)
亲自尝试 »
The sub() Function
The sub() function replaces the matches with the text of your choice:
例子
Replace every white-space character with the number 9:
import re
txt = "The rain in Spain"
x = re.sub("\s", "9", txt)
print(x)
亲自尝试 »
You can control the number of replacements by specifying the count parameter:
例子
Replace the first 2 occurrences:
import re
txt = "The rain in Spain"
x = re.sub("\s", "9", txt, 2)
print(x)
亲自尝试 »
Match Object
A Match Object is an object containing information about the search and the result.
笔记： If there is no match, the value None will be returned, instead of the Match Object.
例子
Do a search that will return a Match Object:
import re
txt = "The rain in Spain"
x = re.search("ai", txt)
print(x) #this will print an object
亲自尝试 »
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
例子
Print the position (start- and end-position) of the first match occurrence.
The regular expression looks for any words that start with an upper case "S":
import re
txt = "The rain in Spain"
x = re.search(r"\bS\w+", txt)
print(x.span())
亲自尝试 »
例子
Print the string passed into the function:
import re
txt = "The rain in Spain"
x = re.search(r"\bS\w+", txt)
print(x.string)
亲自尝试 »
例子
Print the part of the string where there was a match.
The regular expression looks for any words that start with an upper case "S":
import re
txt = "The rain in Spain"
x = re.search(r"\bS\w+", txt)
print(x.group())
亲自尝试 »
笔记： If there is no match, the value None will be returned, instead of the Match Object.
