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.
Share
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:
| 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 |
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 matches at the start/end 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, 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)
Try it Yourself »
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())
Try it Yourself »
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)
Try it Yourself »
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)
Try it Yourself »
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)
Try it Yourself »
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)
Try it Yourself »
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
Try it Yourself »
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())
Try it Yourself »
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)
Try it Yourself »
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.
