Python String Manipulation and Regular Expressions

Formatting Strings: Adjusting Case

• upper
• lower
• capitalize
• title
• swapcase

Formatting Strings: Adjusting Case

• Examples

  >>> fox = "tHe qUICk bROWn fOx'
  >>> fox.upper()
  'THE QUICK BROWN FOX
  >>> fox.title()
  'The Quick Brown Fox'
  >>> fox.swapcase()
  'ThE QuicK BrowN FoX'

Formatting Strings: Adding and Removing Spaces

• strip
• rstrip
• lstrip
• center
• ljust
• rjust

Formatting Strings: Removing Spaces

• Example:

  >>> line = '   stuff   '
  >>> line.strip()
  'stuff'

• The strip methods can remove arbitrary characters

  >>> num = "000000000042"
  >>> num.strip('0')
  '42'

Formatting Strings: Adding Spaces

• Examples:

  >>> line = 'stuff'
  >>> line.center(20)
  '       stuff        '
  >>> line.ljust(20)
  'stuff               '
  >>> '42'.rjust(10, '0')
  '0000000042'

Finding and Replacing Substrings

• find
• rfind
• index
• rindex
• endswith
• startswith
• replace

Finding and Replacing Substrings

• Examples:

  >>> line = 'Hello world'
  >>> line.find('world')
  6
  >>> line.rfind('o')
  7
  >>> line.startswith('He')
  True
  >>> line.replace('o', '---')
  'Hell--- w---rld'

Splitting and Partitioning Strings

• partition
• rpartition
• split
• splitlines
• join

Splitting and Partitioning Strings

• Examples:

  >>> line = 'The quick brown fox jumped over a lazy dog'
  >>> line.partition('fox')
  ('The quick brown ', 'fox', ' jumped over a lazy dog')
  >>> line.split()
  ['The', 'quick', 'brown', 'fox', 'jumped', 'over', 'a', 'lazy', 'dog']
  >>> '--'.join(['1', '2', '3'])
  '1--2--3'

format Basics

• The format method requires that the string have format fields indicated with curly braces.

• The format fields are replaced with objects passed into the format method.

• Example:

  >>> '{} and {}'.format('Alice', 'Bob')
  'Alice and Bob'

format Arguments

• The format fields can be replaced by position or keyword

• Position example:

  >>> '{1} and {0}'.format('Alice', 'Bob')
  'Bob and Alice'

• Keyword example:

  >>> '{a} and {b}'.format(a='Alice', b='Bob')
  'Alice and Bob'

• Mixed example:

  >>> '{b} and {0}'.format('Alice', b='Bob')
  'Bob and Alice'

Formatting Example with format

>>> for x in range(1,11):
...     print('{:2} {:3} {:4}'.format(x,x**2,x**3))
...
 1   1    1
 2   4    8
 3   9   27
 4  16   64
 5  25  125
 6  36  216
 7  49  343
 8  64  512
 9  81  729
10 100 1000

format Specifiers

• An optional ’:’ and format specifier can follow the format field name

• Format specifiers enable greater control over formatted values
  

• Syntax (all are optional): {:[1][2][3][4][5][6][7][8]}

  1. fill and alignment

  2. sign

  3. ’#’ – alternate number form

  4. ’0’ – sign aware zero padding

  5. width

  6. grouping

  7. ‘.’ followed by integer – precision

  8. type

format Specifiers: Width Option

• The width option is a positive integer that specifies padding

• If the width is too small, nothing happens

• Example:

  >>> '{:10}'.format('hello')
  'hello     '
  >>> '{:2}'.format('hello')
  'hello'

format Specifiers: Fill and Alignment

• Alignment options:

  • ’<’: left align

  • ’>’: right align

  • ’=’: pad after sign (if any) but before digits

  • ’^’: center align

• A fill character can optionally be specified before the alignment character

• Example:

  >>> '{:^11}'.format('hello')
  '   hello   '
  >>> '{:>11}'.format('hello')
  '      hello'
  >>> '{:*>11}'.format('hello')
  '******hello'
  >>> '{:-^11}'.format('hello')
  '---hello---'

format Specifiers: Sign option

• The sign option is only valid for number types

  • ’+’: sign should be used for both positive and negative numbers

  • ’-’: sign should be used for only negative numbers (default)

  • ’ ’ (space): leading space for positive numbers and minus sign for negative numbers

• Examples

  >>> '{:+}'.format(123)
  '+123'
  >>> '{: }'.format(123)
  ' 123'
  >>> '{:+}'.format(1.414)
  '+1.414'

format Specifiers: #

• The # option causes a type specific “alternate form” to be used for the conversion

• The # can only be used for integer, float, complex, and Decimal types

• Example

  >>> # print 123 in binary
  >>> '{:b}'.format(123)
  '1111011'
  >>> '{:#b}'.format(123)
  '0b1111011'

format Specifiers: 0

• The 0 (preceding the width option) enables sign aware zero-padding for numeric types

• Example

  >>> '{:010}'.format(123)
  '0000000123'
  >>> '{:010}'.format(1.414)
  '000001.414'

format Specifiers: Grouping Option

• The grouping option specifies a character for separating thousands in numbers

• The grouping option can be either ’_’ or ’,’

• Example

  >>> '{:_}'.format(1000000)
  '1_000_000'
  >>> '{:,}'.format(1000000)
  '1,000,000'

format Specifiers: Precision Option

• The precision option specifies how many digits to be displayed:

  • after the decimal point for fixed point floating point values

  • before and after the decimal point for general floating point values

• Example

  >>> x = math.sqrt(2)
  >>> '{:.2}'.format(x)
  '1.4'
  >>> '{:.2f}'.format(x)
  '1.41'
  >>> '{:.6}'.format(x)
  '1.41421'
  >>> '{:.6f}'.format(x)
  '1.414214'

format Specifiers: Type Option

• String option:

  • ’s’: string

• Common integer options:

  • ’b’: binary

  • ’c’: character

  • ’d’: decimal

  • ’o’: octal

  • ’x’: hex (lower case)

  • ’X’: hex (upper case)

format Specifiers: Type Option (Continued)

• Common float options:

  • ’e’: exponent notation

  • ’E’: exponent notation (upper case E)

  • ’f’: fixed point

  • ’F’: fixed point (upper case NAN and INF)

  • ’g’: general format

  • ’%’: percent (multiply by 100)

format Specifiers: Type Option

• Integer examples

  >>> '{:d}'.format(123)
  '123'
  >>> '{:b}'.format(123)
  '1111011'
  >>> '{:X}'.format(123)
  '7B'

• Floating point examples

  >>> '{:f}'.format(1.414)
  '1.414000'
  >>> '{:E}'.format(1.414)
  '1.414000E+00'
  >>> '{:%}'.format(1.414)
  '141.400000%'

Pattern Matching with Regular Expressions

• Regular expressions are means of flexible pattern matching in strings

• The Python interface to regular expressions is the re module

• Some re methods:
  • compile
  • split
  • match
  • search
  • sub

Pattern Matching with Regular Expressions

• Example

  >>> import re
  >>> regex = re.compile('\s+')
  >>> line = 'the quick brow fox jumped over a lazy dog'
  >>> regex.split(line)
  ['the', 'quick', 'brow', 'fox', 'jumped', 'over', 'a', 'lazy', 'dog']

• The pattern \s+ matches any whitespace character

Pattern Matching with Regular Expressions

• Example: comparing string methods

  >>> line.index('fox')
  15
  >>> regex = re.compile('fox')
  >>> match = regex.search(line)
  >>> match.start()
  15
  >>> line.replace('fox', 'BEAR')
  'the quick brow BEAR jumped over a lazy dog'
  >>> regex.sub('BEAR', line)
  'the quick brow BEAR jumped over a lazy dog'

Basic Regular Expression Syntax

• Simple Strings are matched exactly

  >>> regex = re.compile('ion')
  >>> regex.findall('Great Expectations')
  ['ion']

• Some characters have special meanings:

  . ^ $ * + ? { } [ ] \ | ( )

  • These characters need to be escaped with a backslash (\) to match them

Regular Expression Escape Characters

• Example: the ‘r’ preface indicates a raw string

  >>> regex = re.compile(r'\$')
  >>> regex.findall("the cost is $20')
  ['$']

• Example: strings versus raw strings

  >>> print('a\tb\tc')
  a    b    c
  >>> print(r'a\tb\tc')
  a\tb\tc

Matching Character Groups

• The backslash can be used to give normal characters special meaning

  character	description
  "\d"	match any digit
  "\D"	match any non-digit
  "\s"	match any whitespace
  "\S"	match any non-whitespace
  "\w"	match any alphanumeric char
  "\W"	match any non-alphanumeric char

• Example

  >>> regex = re.compile(r'\w\s\w')
  >>> regex.findall('the fox is 9 years old')
  ['e f', 'x i', 's 9', 's o']

Matching Custom Character Groups

• The square brackets specify a set of characters

  >>> regex = re.compile('[aeiou]')
  >>> regex.split('consequential')
  ['c', 'ns', 'q', '', 'nt', '', 'l']

• The dash can be used to specify a range

  >>> regex = re.compile('[A-Z][0-9]')
  >>> regex.findall('1043879, G2, H6')
  ['G2', 'H6']

Matching Repeated Characters

• Curly braces with a number specify repetition

  >>> regex = re.compile(r'\w{3}')
  >>> regex.findall('The quick brown fox')
  ['The', 'qui', 'bro', 'fox']

Matching Repeated Characters

Matching Repeated Characters

• Example: email address matcher

  email = re.compile(r'[\w.]+@\w+\.[a-z]{3}')

  • "[\w+]" one or more alphanumeric characters or periods
  • "@" at sign
  • "\w+" one or more alphanumeric characters
  • "\." period
  • "[a-z]" exactly three lower case characters