Regular Expression in Python
Regular Expression in Python
Regular expression in Python is also known as RegEx. It is a sequence of characters that defines a search pattern. It is used to examine whether a string contains the specified search pattern or not. To work with regular expressions, built-in Python module called re needs to imported in the script for RegEx.
Meta Characters
In Python, meta characters are the special characters that are interpreted in a different way by the RegEx engine. They are listed below in a table:
| Character | Description | Example |
| [] | This meta character is used to indicate a set of characters. | “[A-Z]” |
| . | This meta character is used to indicate any character but new line. | “He.” |
| ^ | This meta character is used to indicate starting point of the RegEx. | “^Study” |
| $ | This meta character is used to indicate ending point of the RegEx. | “Experts!$” |
| * | This meta character is used to indicate zero or more occurrences of the specified character. | “Hello*” |
| + | This meta character is used to indicate one or more occurrences of the specified character. | “Hello+” |
| {} | This meta character is used to indicate specified number of occurrences of the specified character. | “Hel{2}o” |
| ? | This meta character is used to indicate zero or one occurrence of the specified character. | “Python?” |
| | | This meta character is used to indicate either or condition. | “this|that” |
| () | This meta character is used to indicate grouping of sub-patterns. | “(a|b)ayz” |
| \ | This meta character is used to escape all the escape characters along with the other meta characters. | “\^” |
Regular Expression in Python Example
# Python program to exemplify regular expressions
import re
String = "Studyexperts!"
x = re.search("^S...........!$", String)
if(x):
print("Pattern found.")
else:
print("Pattern not found.")
String = "Studyexperts!."
x = re.search("^S...........!$", String)
if(x):
print("Pattern found.")
else:
print("Pattern not found.")
Output:
Pattern found. Pattern not found.
Special Sequences
| Sequence | Description | Example |
| \A | This special sequence is used to indicate that the specified characters are at the beginning of the string. | “\AYou” |
| \b | This special sequence is used to indicate that the specified characters are at the beginning or at the end of the string. | “\bthat”
“that\b” |
| \B | This special sequence is used to indicate that the specified characters are present but not at the beginning or at the end of the string. | “\Bthat”
“that\B” |
| \d | This special sequence is used to indicate that the string contains digits. | “\d” |
| \D | This special sequence is used to indicate that the string does not contain digits. | “\D” |
| \s | This special sequence is used to indicate that the string contains a white space character. | “\s” |
| \S | This special sequence is used to indicate that the string does not contain a white space character. | “\S” |
| \w | This special sequence is used to indicate that the string contains any word characters which are characters from a to Z, digits from 0-9, and the underscore _ character. | “\w” |
| \W | This special sequence is used to indicate that the string does not contain any word characters. | “\W” |
| \Z | This special sequence is used to indicate that the specified characters are at the end of the string. | “StudyExperts!\Z” |
Sets
A set is a collection of characters placed inside a pair of square brackets with a special meaning.
| Set | Description |
| [abc] | This set is used to indicate that a string should have one of the specified characters (a, b, or c). |
| [a-d] | This set is used to indicate that a string should have one of the specified characters (a, b, c, or d). |
| [^abc] | This set is used to indicate that a string should have any character except for a, b, and c. |
| [123] | This set is used to indicate that a string should have one of the specified digits (1, 2, or 3). |
| [0-9] | This set is used to indicate that a string should have any of the digits between 0 and 9. |
| [1-8][0-9] | This set is used to indicate that a string should have a two digit-number between 10 and 89. |
| [a-zA-Z] | This set is used to indicate that a string should have any alphabetic character between a and z, either lowercase or uppercase. |
| [+] | In sets, +, *, ., |, (), $,{} has no special meaning, so [+] means: return a match for any character in the string. |