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:
RegEx in Python
When you have imported the re module, you can start using regular expressions:
Example
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]
Try it Yourself »
RegEx Functions
The re module offers a set of functions that allows us to search a string for a match:
| 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:
| [] | 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:
| \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:
| [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, and 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.
Example
Print a list of all matches:
import re
txt = "The rain in Spain"
x = re.findall["ai", txt]
print[x]
Try it Yourself »
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:
import re
txt = "The rain in Spain"
x = re.findall["Portugal", txt]
print[x]
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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:
Example
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[]]
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If no matches are found, the value None is returned:
Example
Make a search that returns no match:
import re
txt = "The rain in Spain"
x = re.search["Portugal", txt]
print[x]
Try it Yourself »
The split[] Function
The split[] function returns a list where the string has been split at each match:
Example
Split at each white-space character:
import re
txt = "The rain in Spain"
x = re.split["\s", txt]
print[x]
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You can control the number of occurrences by specifying the maxsplit
parameter:
Example
Split the string only at the first occurrence:
import re
txt = "The rain in Spain"
x = re.split["\s", txt, 1]
print[x]
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The sub[] Function
The sub[] function replaces the matches with the text of your choice:
Example
Replace every white-space character with the number 9:
import re
txt = "The rain in Spain"
x = re.sub["\s", "9", txt]
print[x]
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You can control the number of replacements by specifying the count parameter:
Example
Replace the first 2 occurrences:
import re
txt = "The rain in Spain"
x = re.sub["\s", "9", txt, 2]
print[x]
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Match Object
A Match Object is an object containing information about the search and the result.
Note: If there is no match, the value None will be returned, instead of the Match Object.
Example
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
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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":
import re
txt = "The rain in Spain"
x = re.search[r"\bS\w+", txt]
print[x.span[]]
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Example
Print the string passed into the function:
import re
txt = "The rain in Spain"
x = re.search[r"\bS\w+", txt]
print[x.string]
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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":
import re
txt = "The rain in Spain"
x = re.search[r"\bS\w+", txt]
print[x.group[]]
Try it Yourself »
Note: If there is no match, the value None will be returned, instead of the Match Object.