ReactCHALLENGES

5. Substitute

re.sub() replaces matches with new text and returns a new string:

  • pattern — the regex to find.
  • replacement — the text that replaces each match.
  • string — the original text; it is not changed, because strings are immutable.
  • count — optional; pass it by name as count=1. Default 0 replaces all matches; count=1 replaces only the first.
  • flags — optional, same flags as search and findall, for example re.IGNORECASE.

The replacement can refer to captured groups: \1 is the text of group 1, \2 is group 2, and so on. Use a raw string for the replacement so \1 reaches re intact:

Coming from JavaScript? re.sub is text.replace(regex, replacement), but Python replaces every match by default. JS replace with a string pattern replaces only the first and needs replaceAll or a /g regex to replace all.

#01

Replace every match

Import re and call re.sub() with the pattern "cat", the replacement "dog" and the text "cat cat cat". Print the new string.

Print exactly:

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#02

Replace only the first match

Use the same pattern, replacement and text, but pass count=1 to re.sub(). Only the first cat is replaced.

Print exactly:

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#03

Replace ignoring case

re.sub() accepts the same flags as the other functions. Replace "cat" with "dog" in "Cat cat CAT" using flags=re.IGNORECASE. All three spellings match.

Print exactly:

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#04

Reorder with groups

Use the pattern r"(Ada) (Lovelace)" on "Ada Lovelace". Make the replacement the raw string r"\2, \1": group 2, a comma and a space, then group 1. Print the new string.

Print exactly:

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