Master Python Regex: Essential re Module Functions and Patterns Explained

While writing a crawler, I recalled a great article I read when first learning regular expressions and found it again.

re Module

re.search

We often use match = re.search(pat, str). Since it may not find a match, re.search() is usually followed by an if statement.

re.match

re.match

is similar to re.search, but it matches only from the beginning of the string.

Common Regex Symbols

a, X, 9 : match themselves; metacharacters like . ^ $ * + ? { } [ ] \ | ( ) have special meanings.

. : matches any character except '\n'.

\w : matches word characters [a-zA-Z0-9_].

\W : matches non‑word characters.

\b : matches a word boundary.

\s : matches a whitespace character (space, newline, tab, etc.).

\S : matches a non‑whitespace character.

\t, \n, \r : match tab, newline, carriage return.

\d : matches digits [0-9].

^ : matches the start of a string.

$ : matches the end of a string.

Quantifiers

‘+’ means one or more times, ‘*’ means zero or more times, ‘?’ means zero or one time.

Character Classes []

Square brackets indicate a set of characters, so [abc] matches ‘a’ or ‘b’ or ‘c’.

Group Extraction with ()

Parentheses can capture parts of a match. For example, the pattern r'([\w.-]+)@([\w.-]+)' can extract the username and hostname of an email; after a successful match, match.group(1) and match.group(2) return the captured parts while match.group() returns the whole match.

findall and Groups

Combining () with findall() returns a list of tuples when one or more groups are present.

Adding ^ to re.search makes it behave like re.match.

re.sub

re.sub(pat, replacement, str)

searches for substrings matching pat in str and replaces them with replacement. The replacement can include \1 or \2 to refer to captured groups, enabling partial substitution.