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. Python provides the built-in function called re which can be used to work with Regular Expressions. The below syntax is used to load re module.
import re
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 Math 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 |
|---|---|---|
| [] | A set of characters | “[a-m]” |
| \ | Signals a special sequence (can also be used to escape special characters) | “\d” |
| . | Any character (except newline character) | “he..o” |
| ^ | Starts with | “^hello” |
| $ | Ends with | “world$” |
| * | Zero or more occurrences | “aix*” |
| + | One or more occurrences | “aix+” |
| {} | Exactly the specified number of occurrences | “al{2}” |
| | | Either or | “falls|stays” |
| () | 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 | Example |
|---|---|---|
| \A | Returns a match if the specified characters are at the beginning of the string | “\AThe” |
| \b | Returns a match where the specified characters are at the beginning or at the end of a word | r”\bain” r”ain\b” |
| \B | Returns a match where the specified characters are present, but NOT at the beginning (or at the end) of a word | r”\Bain” r”ain\B” |
| \d | Any digit, equivalent to [0-9] | “\d” |
| \D | Any non-digit, equivalent to [^ 0-9] | “\D” |
| \s | Any whitespace, equivalent to [ \t\n\r\f\v] | “\s” |
| \S | Any non-whitespace, equivalent to [^ \t \n \r \f \v] | “\S” |
| \w | Any Alphanumeric character, equivalent to [a-zA-Z0-9_] | “\w” |
| \W | Any Non-alphanumeric character, equivalent to [^ a-zA-Z0-9_] | “\W” |
| \Z | Returns a match if the specified characters are at the end of the string | “Spain\Z” |
. . .
Sets
A set is a set of characters inside a pair of square brackets [] with a special meaning:
| Set | Description |
|---|---|
| [arn] | Returns a match where one of the specified characters (a, r, or n) are present |
| [a-n] | Returns a match for any lower case character, alphabetically between a and n |
| [^arn] | Returns a match for any character EXCEPT a, r, and n |
| [0123] | Returns a match where any of the specified digits (0, 1, 2, or 3) are present |
| [0-9] | Returns a match for any digit between 0 and 9 |
| [0-5][0-9] | Returns a match for any two-digit numbers from 00 and 59 |
| [a-zA-Z] | Returns a match for any character alphabetically between a and z, lower case OR upper case |
| [+] | In sets, +, *, . , | , () , $ , {} has no special meaning, so [+] means: return a match for any + character in the string |
. . .
The findall() Function
The findall() function returns a list containing all matches. If no match found, return an empty list.
In [1]:
import re
str = "the weather is very pleasant."
x = re.findall("ea", str)
y = re.findall("a very", str)
In [2]: x
Out[2]: ['ea', 'ea']
In [3]: y
Out[3]: []
. . .
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.
In [4]:
import re
str = "the weather is very pleasant."
x = re.search("we", str)
In [5]: x
Out[5]: <_sre.SRE_Match object; span=(4, 6), match='we'>
In [6]: x.start()
Out[6]: 4
In [7]: x.end()
Out[7]: 6
. . .
The split() Function
The split() function returns a list where the string has been split at each match. we can control the number of occurrences by specifying the maxsplit parameter
In [8]:
import re
str = "the weather is very pleasant."
x = re.split("\s", str)
y = re.split("\s", str, 1)
In [9]: x
Out[9]: ['the', 'weather', 'is', 'very', 'pleasant.']
In [10]: y
Out[10]: ['the', 'weather is very pleasant.']
. . .
The sub() Function
The sub() function replaces the matches with the text of your choice. You can control the number of replacements by specifying the count parameter.
In [11]:
import re
str = "the weather is very pleasant."
x = re.sub("\s", "_", str)
y = re.sub("\s", "_", str, 2)
In [12]: x
Out[12]: 'the_weather_is_very_pleasant.'
In [13]: y
Out[13]: 'the_weather_is very pleasant.'
. . .
Match Object
A Match Object is an object containing information about the search and the result. 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
In [14]:
import re
str = "the weather is very pleasant."
x = re.search("weather", str)
In [15]: x.span()
Out[15]: (4, 11)
In [16]: x.string
Out[16]: 'the weather is very pleasant.'
In [17]: x.group()
Out[17]: 'weather'
. . .