Programming on Python. Lecture 7

Intro to Python for Data Science
Regular Expression

WHAT IS A REGULAR EXPRESSION?
A Regular Expression (RegEx) is a sequence of characters that defines
a search pattern.
For example,
^a...s$
The above code defines a RegEx pattern.
The pattern is: any five letter string starting with a and ending with s.

A pattern defined using RegEx can be used to match against a string.

Python has a module named re to work with RegEx. Here's an example:
import re
pattern = '^a...s$’
test_string = 'abyss’
result = re.match(pattern, test_string)
if result:
print("Search successful.")
else: print("Search unsuccessful.")
Here, we used re.match() function to search pattern within the test_string.
The method returns a match object if the search is successful. If not, it returns None.

THERE ARE OTHER SEVERAL FUNCTIONS DEFINED IN THE RE MODULE TO WORK WITH REGEX. BEFORE WE EXPLORE THAT, LET'S LEARN ABOUT REGULAR EXPRESSIONS.

SPECIFY PATTERN USING REGEX
To specify regular expressions, metacharacters are used.
In the previous example, ^ and $ are metacharacters.

METACHARACTERS
METACHARACTERS ARE CHARACTERS THAT ARE INTERPRETED IN A SPECIAL WAY BY A REGEX ENGINE. HERE'S A LIST OF METACHARACTERS:
[] . ^ $ * + ? {} () \ |

METACHARACTERS
[] - Square brackets
Square brackets specifies a set of characters you wish to match.
Here, [abc] will match if the string you are trying to match contains any of the a, b or c.

METACHARACTERS
You can also specify a range of characters using - inside square brackets.
[a-e] is the same as [abcde].
[1-4] is the same as [1234].
[0-39] is the same as [01239].
You can complement (invert) the character set by using caret ^ symbol at
the start of a square-bracket.
[^abc] means any character except a or b or c.
[^0-9] means any non-digit character.

METACHARACTERS
. - Period
A period matches any single character (except newline '\n').

METACHARACTERS
^ - Caret
The caret symbol ^ is used to check if a string starts with a certain character.

METACHARACTERS
$ - Dollar
The dollar symbol $ is used to check if a string ends with a certain character.

METACHARACTERS
* - Star
The star symbol * matches zero or more occurrences of the pattern left to it.

METACHARACTERS
+ - Plus
The plus symbol + matches one or more occurrences of the pattern left to it.

METACHARACTERS
? - Question Mark
The question mark symbol ? matches zero or one occurrence of the pattern left to it.

METACHARACTERS
{} - Braces
Consider this code: {n,m}.
This means at least n, and at most m repetitions of the pattern left to it.

METACHARACTERS
Let's try one more example. This RegEx [0-9]{2, 4} matches at least 2 digits but not more than 4 digits

METACHARACTERS
| - Alternation
Vertical bar | is used for alternation (or operator).
Here, a|b match any string that contains either a or b

METACHARACTERS
() - Group
Parentheses () is used to group sub-patterns. For example, (a|b|c)xz match any string that
matches either a or b or c followed by xz

METACHARACTERS
\ - Backslash
Backlash \ is used to escape various characters including all metacharacters.
For example,
\$a match if a string contains $ followed by a. Here, $ is not interpreted by a RegEx engine in a special way.
If you are unsure if a character has special meaning or not, you can put \ in front of it. This makes sure the character is not treated in a special way.

SPECIAL SEQUENCES
Special sequences make commonly used patterns easier to write. Here's a list of special sequences:
\A - Matches if the specified characters are at the start of a string.

SPECIAL SEQUENCES
\b - Matches if the specified characters are at the beginning or end of a word.

SPECIAL SEQUENCES
\B - Opposite of \b. Matches if the specified characters are not at the beginning or end of a word.

SPECIAL SEQUENCES
\d - Matches any decimal digit. Equivalent to [0-9]
\D - Matches any non-decimal digit. Equivalent to [^0-9]

SPECIAL SEQUENCES
\s - Matches where a string contains any whitespace character. Equivalent to [ \t\n\r\f\v].
\S - Matches where a string contains any non-whitespace character. Equivalent to [^ \t\n\r\f\v].

SPECIAL SEQUENCES
\w - Matches any alphanumeric character (digits and alphabets). Equivalent to [a-zA-Z0-9_].
By the way, underscore _ is also considered an alphanumeric character.
\W - Matches any non-alphanumeric character. Equivalent to [^a-zA-Z0-9_]

SPECIAL SEQUENCES
\Z - Matches if the specified characters are at the end of a string.

SPECIAL SEQUENCES
Tip: To build and test regular expressions, you can use RegEx tester tools such as regex101.com. This tool not only helps you in creating regular expressions, but it also helps you learn it.
Now we understand the basics of RegEx, let’s learn how to use RegEx in Python code.

PYTHON REGEX
Python has a module named re to work with regular expressions.
To use it, we need to import the module.
import re
The module defines several functions and constants to work with RegEx.

PYTHON REGEX
re.findall()
The re.findall() method returns a list of strings containing all matches.
Example 1: re.findall()
# Program to extract numbers from a string
import re
string = 'hello 12 hi 89. Howdy 34’
pattern = '\d+’
result = re.findall(pattern, string)
print(result) # Output: ['12', '89', '34’]
If the pattern is not found, re.findall() returns an empty list.

PYTHON REGEX
re.split()
The re.split method splits the string where there is a match and returns a list of strings
where the splits have occurred.
Example 2: re.split()
import re
string = 'Twelve:12 Eighty nine:89.’
pattern = '\d+’
result = re.split(pattern, string)
print(result)
# Output: ['Twelve:', ' Eighty nine:', '.’]
If the pattern is not found, re.split() returns a list containing the original string.

PYTHON REGEX
You can pass maxsplit argument to the re.split() method. It's the maximum number of splits that will
occur.
import re
string = 'Twelve:12 Eighty nine:89 Nine:9.’
pattern = '\d+' # maxsplit = 1 # split only at the first occurrence
result = re.split(pattern, string, 1)
print(result)
# Output: ['Twelve:', ' Eighty nine:89 Nine:9.’]
By the way, the default value of maxsplit is 0; meaning all possible splits.

PYTHON REGEX
re.sub()
The syntax of re.sub() is:
re.sub(pattern, replace, string)
The method returns a string where matched occurrences are replaced with the content of replace variable.

PYTHON REGEX
Example 3: re.sub()
# Program to remove all whitespaces
import re
# multiline string
string = 'abc 12\ de 23 \n f45 6’
# matches all whitespace characters
pattern = '\s+’
# empty string
replace = ‘’
new_string = re.sub(pattern, replace, string)
print(new_string)
# Output: abc12de23f456
If the pattern is not found, re.sub() returns the original string.

PYTHON REGEX
You can pass count as a fourth parameter to the re.sub() method.
If omitted, it results to 0. This will replace all occurrences.
import re
# multiline string
string = 'abc 12\ de 23 \n f45 6’
# matches all whitespace characters
pattern = '\s+’
replace = ‘’
new_string = re.sub(pattern, replace, string, 1)
print(new_string)
# Output:
# abc12\ de 23
# f45 6

PYTHON REGEX
re.subn()
The re.subn() is similar to re.sub() expect it returns a tuple of 2 items containing the
new string and the number of substitutions made.
Example 4: re.subn()
# Program to remove all whitespaces
import re
# multiline string
string = 'abc 12\ de 23 \n f45 6’
# matches all whitespace characters
pattern = '\s+’
# empty string
replace = ‘’
new_string = re.subn(pattern, replace, string)
print(new_string)
# Output: ('abc12de23f456', 4)

PYTHON REGEX
re.search()
The re.search() method takes two arguments: a pattern and a string.
The method looks for the first location where the RegEx pattern produces a match
with the string.
If the search is successful, re.search() returns a match object; if not, it returns None.
match = re.search(pattern, str)

PYTHON REGEX
Example 5: re.search()
import re
string = "Python is fun"
# check if 'Python' is at the beginning
match = re.search('\APython', string)
if match:
print("pattern found inside the string")
else:
print("pattern not found")
# Output: pattern found inside the string
Here, match contains a match object.

MATCH OBJECT
You can get methods and attributes of a match object using dir() function.
Some of the commonly used methods and attributes of match objects are:
match.group()
The group() method returns the part of the string where there is a match.
Example 6: Match object
import re string = '39801 356, 2102 1111’
# Three digit number followed by space followed by two digit number
pattern = '(\d{3}) (\d{2})’
# match variable contains a Match object.
match = re.search(pattern, string)
if match:
print(match.group())
else: print("pattern not found")
# Output: 801 35
Here, match variable contains a match object.

MATCH OBJECT
match.start(), match.end() and match.span()
The start() function returns the index of the start of the matched substring.
Similarly, end() returns the end index of the matched substring.
>>> match.start()
2
>>> match.end()
8
The span() function returns a tuple containing start and end index of the matched part.
>>> match.span()
(2, 8)

MATCH OBJECT
match.re and match.string
The re attribute of a matched object returns a regular expression object.
Similarly, string attribute returns the passed string.
>>> match.re
re.compile('(\\d{3}) (\\d{2})’)
>>> match.string
'39801 356, 2102 1111'

USING R PREFIX BEFORE REGEX
When r or R prefix is used before a regular expression, it means raw string. For example, '\n' is a new line whereas r'\n' means two characters: a backslash \ followed by n.
Backlash \ is used to escape various characters including all metacharacters. However, using r prefix makes \ treat as a normal character.
Example 7: Raw string using r prefix
import re
string = '\n and \r are escape sequences.’
result = re.findall(r'[\n\r]', string)
print(result)
# Output: ['\n', '\r']