Regular Expressions#

Section Title: Regular Expressions

Regular expressions, commonly known as regex, are a powerful tool for working with strings and text.

  • They are widely used when handling files containing long texts or strings to search for specific patterns or modify the original text.

  • Instead of writing lengthy programs with numerous iterations and conditional statements, regular expressions allow you to accomplish the same tasks in a concise and readable manner.

  • This makes them an efficient and effective way to process and manipulate text.

  • The module for working with regular expressions in Python is called re and should be imported using:

import re

Characters#

The following characters are commonly used to define patterns for finding matches:

  • Caret (^): Matches the start of a string.

    • ‘^x’ matches ‘xy’ at the beginning.

  • Dollar Sign ($): Matches the end of a string.

    • ‘x$’ matches ‘yx’ at the end.

  • Dot (.): Matches any character except a newline.

    • ‘x.y’ matches ‘xay’, ‘xby’

  • Asterisk (*): Matches zero or more repetitions of the preceding character.

    • ‘x*’ matches ‘’, ‘x’, ‘xx’, ‘xxx’

  • Plus (+): Matches one or more repetitions of the preceding character.

    • ‘x+’ matches ‘x’, ‘xx’, ‘xxx’

  • Question Mark (?): Matches zero or one occurrence of the preceding character.

    • ‘xy?’ matches ‘x’ or ‘xy’.

  • Pipe (|): Acts as an OR operator.

    • ‘xyz|yzt’ matches ‘xyz’ or ‘yzt’.

  • Backslash (\): Escapes special characters or introduces special sequences.

    • \. matches a literal dot.

  • Parentheses (()): Groups patterns and captures matches.

    • ‘(xyz)+’ matches one or more repetitions of ‘xyz’.

  • Square Brackets ([...]): Matches any one character inside the brackets.

    • 0-9: all digits

    • A-Za-z: all letters

    • A-Z: uppercase letters

    • a-z: lowercase letters

    • [^...]: except the characters in the square bracket

  • \d: Matches any digit (0-9).

    • ‘\d’ matches ‘1’,’2’,’3’,..,’9’

  • \D: Matches any non-digit.

    • ‘\D’ matches ‘x’, ‘c’, ‘!’

  • \s: Matches any whitespace character.

    • ‘\s’ matches spaces, tabs

  • \S:Matches any non-whitespace character.

    • ‘\S’ matches ‘x’, ‘1’

  • \w: Matches any alphanumeric character (letters, digits, or underscore).

    • ‘\w’ matches ‘x’, ‘1’, ‘_’.

  • \W: Matches any non-alphanumeric character.

    • ‘\W’ matches ‘%’, ‘#’

findall()#

  • Purpose: Extracts all occurrences of a specified pattern in a string.

  • Return Value: A list of non-overlapping matches for the pattern in the string.

  • Order of Output: Matches are returned in the order they appear in the string.

This function is useful for identifying and retrieving all instances of a pattern efficiently.

help(re.findall)

Help on function findall in module re:

findall(pattern, string, flags=0)
    Return a list of all non-overlapping matches in the string.
    
    If one or more capturing groups are present in the pattern, return
    a list of groups; this will be a list of tuples if the pattern
    has more than one group.
    
    Empty matches are included in the result.

Caret#

  • Starts with ‘a’

re.findall('^a' , 'abcd')

['a']

re.findall('^a' , 'bcd')

[]

  • Starts with ‘b’

re.findall('^b' , 'abcd')

[]

Dollar sign#

  • Ends with ‘d’

re.findall('d$' , 'abcd')

['d']

  • Ends with ‘a’

re.findall('a$' , 'bcd')

[]

  • Ends with ‘b’

re.findall('b$' , 'abcd')

[]

Dot#

  • Any character (except newline) between between ‘a’ and ‘c’

re.findall('a.c' , 'afc')

['afc']

re.findall('a.c' , 'bcd')

[]

  • Any two characters (except newline) between between ‘a’ and ‘c’

re.findall('a..c' , 'axyc')

['axyc']

Asterisk#

  • Zero or more ‘a’

re.findall('a*' , 'axyc')

['a', '', '', '', '']

  • Zero or more ‘c’

re.findall('c*' , 'axyc')

['', '', '', 'c', '']

  • ‘a’ followed by one or more ‘x’

re.findall('ax*' , 'axyc')

['ax']

Plus#

  • One or more ‘a’

re.findall('a+' , 'axyc')

['a']

  • One or more ‘c’

re.findall('c+' , 'axyc')

['c']

  • ‘a’ followed by one or more ‘x’

re.findall('ax+' , 'axyc')

['ax']

Question Mark#

  • Zero or more ‘a’

re.findall('a?' , 'axyc')

['a', '', '', '', '']

  • Zero or more ‘c’

re.findall('c?' , 'axyc')

['', '', '', 'c', '']

  • ‘a’ followed by zero or more ‘x’

re.findall('ax?' , 'axyc')

['ax']

Pipe#

  • ‘x’ or ‘y’

re.findall('x|y' , 'axyc')

['x', 'y']

  • ‘ax’ or ‘yd’

re.findall('ax|yd' , 'axyc')

['ax']

Backslash#

  • ‘a.c’ matches ‘a.c’

re.findall('a\.c' , 'abcda.c')

['a.c']

  • ‘a..c’ matches ‘a.(any character)c’

re.findall('a\..c' , 'abcda.fc')

['a.fc']

Paranthesis#

  • Matches one or more repetitions of ‘ab’

re.findall('(ab)+' , 'abcdab')

['ab', 'ab']

  • Matches one or more repetitions of ‘a(any character)b’

re.findall('(a.b)+' , 'axbcdayb')

['axb', 'ayb']

  • Paranthesis also used to extract only the part of the match inside the parentheses.

  • Starts with ‘c’ and ends with ‘d’, with the match taking the form ‘a(any character)d’.

re.findall('c(a.b)d' , 'apbxcavbd')

['avb']

Square Brackets#

  • All digits

re.findall('[0-9]', '1abcd')

['1']

  • Non-digits

re.findall('[^0-9]', '1abcd')

['a', 'b', 'c', 'd']

  • All letters

re.findall('[A-Za-z]', '1abcd')

['a', 'b', 'c', 'd']

  • All non-letters

re.findall('[^A-Za-z]', '1ab!cd')

['1', '!']

  • Match the characters ‘d’, ‘n’, and ‘k’

re.findall('[dnk]', '1abd!cnd')

['d', 'n', 'd']

Digits#

re.findall('\d', '1ab!cd')

['1']

Non-digits#

re.findall('\D', '1ab!cd')

['a', 'b', '!', 'c', 'd']

Whitespace#

re.findall('\s', '1a\nb!\tc d')

['\n', '\t', ' ']

Non-Whitespace#

re.findall('\S', '1a\nb!\tc d')

['1', 'a', 'b', '!', 'c', 'd']

Alphanumeric#

re.findall('\w', '1a\nb!\tc d')

['1', 'a', 'b', 'c', 'd']

Non-Alphanumeric#

re.findall('\W', '1a%&@\nb!\tc d')

['%', '&', '@', '\n', '!', '\t', ' ']