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Delving Deeper

Regular Expressions

Escape Codes

An even more compact representation uses escape codes for several predefined character sets. The escape codes recognized by re are listed in the table below.

Regular Expression Escape Codes
Code Meaning
\d a digit
\D a non-digit
\s whitespace (tab, space, newline, etc.)
\S non-whitespace
\w alphanumeric
\W non-alphanumeric

Note

Escapes are indicated by prefixing the character with a backslash (\). Unfortunately, a backslash must itself be escaped in normal Python strings, and that results in difficult-to-read expressions. Using raw strings, which are created by prefixing the literal value with r, eliminates this problem and maintains readability.


# re_escape_codes.py

from re_test_patterns import test_patterns

test_patterns(
    'A prime #1 example!',
    [(r'\d+', 'sequence of digits'),
     (r'\D+', 'sequence of non-digits'),
     (r'\s+', 'sequence of whitespace'),
     (r'\S+', 'sequence of non-whitespace'),
     (r'\w+', 'alphanumeric characters'),
     (r'\W+', 'non-alphanumeric')],
)

These sample expressions combine escape codes with repetition to find sequences of like characters in the input string.


$ python3 re_escape_codes.py

'\d+' (sequence of digits)

  'A prime #1 example!'
  .........'1'

'\D+' (sequence of non-digits)

  'A prime #1 example!'
  'A prime #'
  ..........' example!'

'\s+' (sequence of whitespace)

  'A prime #1 example!'
  .' '
  .......' '
  ..........' '

'\S+' (sequence of non-whitespace)

  'A prime #1 example!'
  'A'
  ..'prime'
  ........'#1'
  ...........'example!'

'\w+' (alphanumeric characters)

  'A prime #1 example!'
  'A'
  ..'prime'
  .........'1'
  ...........'example'

'\W+' (non-alphanumeric)

  'A prime #1 example!'
  .' '
  .......' #'
  ..........' '
  ..................'!'

To match the characters that are part of the regular expression syntax, escape the characters in the search pattern.

# re_escape_escapes.py

from re_test_patterns import test_patterns

test_patterns(
    r'\d+ \D+ \s+',
    [(r'\\.\+', 'escape code')],
    )

The pattern in this example escapes the backslash and plus characters, since both are meta-characters and have special meaning in a regular expression.

$ python3 re_escape_escapes.py

'\\.\+' (escape code)

  '\d+ \D+ \s+'
  '\d+'
  .....'\D+'
  ..........'\s+''