Delving Deeper

Regular Expressions

Input with Multiple Lines

Two flags affect how searching in multi-line input works: MULTILINE and DOTALL. The MULTILINE flag controls how the pattern matching code processes anchoring instructions for text containing newline characters. When multiline mode is turned on, the anchor rules for ^ and $ apply at the beginning and end of each line, in addition to the entire string.

# re_flags_multiline.py

import re

text = 'This is some text -- with punctuation.\nA second line.'
pattern = r'(^\w+)|(\w+\S*$)'
single_line = re.compile(pattern)
multiline = re.compile(pattern, re.MULTILINE)

print('Text:\n  {!r}'.format(text))
print('Pattern:\n  {}'.format(pattern))
print('Single Line :')
for match in single_line.findall(text):
    print('  {!r}'.format(match))
print('Multline    :')
for match in multiline.findall(text):
    print('  {!r}'.format(match))

The pattern in the example matches the first or last word of the input. It matches line. at the end of the string, even though there is no newline.

$ python3 re_flags_multiline.py

Text:
  'This is some text -- with punctuation.\nA second line.'
Pattern:
  (^\w+)|(\w+\S*$)
Single Line :
  ('This', '')
  ('', 'line.')
Multline    :
  ('This', '')
  ('', 'punctuation.')
  ('A', '')
  ('', 'line.')

DOTALL is the other flag related to multiline text. Normally, the dot character (.) matches everything in the input text except a newline character. The flag allows the dot to match newlines as well.

# re_flags_dotall.py
import re

text = 'This is some text -- with punctuation.\nA second line.'
pattern = r'.+'
no_newlines = re.compile(pattern)
dotall = re.compile(pattern, re.DOTALL)

print('Text:\n  {!r}'.format(text))
print('Pattern:\n  {}'.format(pattern))
print('No newlines :')
for match in no_newlines.findall(text):
    print('  {!r}'.format(match))
print('Dotall      :')
for match in dotall.findall(text):
    print('  {!r}'.format(match))

Without the flag, each line of the input text matches the pattern separately. Adding the flag causes the entire string to be consumed.

$ python3 re_flags_dotall.py

Text:
  'This is some text -- with punctuation.\nA second line.'
Pattern:
  .+
No newlines :
  'This is some text -- with punctuation.'
  'A second line.'
Dotall      :
  'This is some text -- with punctuation.\nA second line.'