Regular Expressions in Python

Regular Expression Functions and Methods

• re.compile() creates Regex objects.
• Raw strings (prefixed with 'r') are often used for Regex patterns to avoid backslash escapes.
• search() returns a Match object if a match is found; otherwise, it returns None.
• To get the matched strings from a Match object, use the group() method. For instance, match.group(0) returns the entire match, match.group(1) returns the first captured group, and so on.

Regex Specifics with Examples

• r'(\d\d\d)-(\d\d\d-\d\d\d\d)': This regex matches a phone number with the format ###-###-####.
  • group(0) covers the entire match (e.g., '123-456-7890').
  • group(1) covers the first set of three digits (e.g., '123').
  • group(2) covers the second set of three digits followed by four digits (e.g., '456-7890').
• To match literal parentheses or periods, escape them with a backslash (e.g., \(, \) or \.)
• findall() returns a list of strings if the regex doesn't contain capturing groups, or a list of tuples of strings if capturing groups are present.
• | signifies OR (alternation) within a regular expression
• ? signifies zero or one occurrences of the preceding element, or non-greedy match
• + signifies one or more occurrences of the preceding element.
• * signifies zero or more occurrences of the preceding element.
• {3} signifies exactly three occurrences of the preceding element.
• {3,5} signifies between three and five occurrences of the preceding element.
• \d matches any digit (0-9)
• \w matches any alphanumeric character (a-z, A-Z, 0-9, _)
• \s matches any whitespace character (space, tab, newline).
• \D, \W, \S match the complements of \d, \w and \s respectively (non-digits, non-alphanumerics, non-whitespace)
• .* is greedy (matches as much text as possible); .*? is non-greedy (matches as little text as possible).
• [0-9a-z] matches any lowercase letter or digit.
• To make a regex case-insensitive, use the re.IGNORECASE (or re.I) flag.
• . normally matches any character except a newline. re.DOTALL (or re.S) makes . match any character including a newline.
• numRegex = re.compile(r'\d+'): The sub() method with this regex can replace all sequences of digits with 'X'.
• re.VERBOSE allows for more readable regexes by ignoring whitespace and adding comments.
• Regex for numbers with commas every three digits: r'\d{1,3}(,\d{3})*
• Regex for names with a capitalized first name followed by last name "Watanabe": r'[A-Z][a-z]+\sWatanabe'
• Regex for sentences matching specific conditions with case-insensitive: r'(Alice|Bob|Carol)\s(eats|pets|throws)\s(apples|cats|baseballs)\.', case-insensitive.