Regex and Wildcard

RegEx and Wildcard #

Wildcards #

Wildcards are characters used to represent any number of characters or specific character sets within a string. They’re commonly used in search patterns for file names and text.

For instance, in UNIX-like operating systems, the '*' and '?' characters are popular wildcards. Here’s a brief explanation of these two wildcard characters:

  • Asterisk (*): Represents zero or more characters, often used for matching multiple files or text patterns.
  • Question mark (?): Represents exactly one character, typically used when you want to match a pattern with a variable single character.

RegEx #

Regular expressions are a powerful tool for pattern matching and searching within text. They allow you to define a search pattern and apply it to a string to find, extract, or modify the desired text. RegEx is available in most programming languages, such as Python, JavaScript, Java, and Perl.

Basic RegEx Concept #

  • Literal characters: Matches the exact character(s) in the string.

    • Example: The RegEx pattern "abc" would match the substring "abc" in the string "abcdef".

  • Metacharacters: Special characters that have a specific meaning in the context of a regular expression.

    • Example: The dot (.) is a metacharacter that matches any single character except a newline.

  • Character classes: Enclosed in square brackets [], they define a set of characters to match.

    • Example: The RegEx pattern "[aeiou]" would match any single vowel in the string "hello world".

RegEx Quantifiers #

Zero or one: A question mark (?) indicates that the preceding character or group may appear zero or one time.

Example: The RegEx pattern “colou?r” matches both “color” and “colour”.

Zero or more: An asterisk (*) indicates that the preceding character or group may appear zero or more times.

Example: The RegEx pattern "ab*c" matches “ac”, “abc”, “abbc”, and so on.

One or more: A plus sign (+) indicates that the preceding character or group must appear at least once.

Example: The RegEx pattern "ab+c" matches “abc”, “abbc”, but not “ac”.

Specific number: Curly braces {} contain a number to indicate how many times the preceding character or group should appear.

Example: The RegEx pattern "ab{3}c" matches “abbbc”, but not “abc” or “abbc”.

Anchors #

Start of the string: The caret (^) symbol indicates that the pattern must start at the beginning of the string.

Example: The RegEx pattern “^abc” matches “abcde”, but not “xabcde”.

Note: In [], ^, on the other hand, ONLY means “not the following” when inside AND at the start of [].

To clarify, therefore:

  • [^abc] » not a, b or c
  • [ab^cd] » a, b, ^ (character), c or d
  • \^ » an actual ^ character

End of the string: The dollar sign ($) indicates that the pattern must appear at the end of the string.

Example: The RegEx pattern “abc$” matches “xabc”, but not “xabcx”.

Grouping and Capturing #

Vertical bar (|): Represents an OR operator, allowing a match for either the expression on the left or the right.

Example: The RegEx pattern “abc|xyz” matches either “abc” or “xyz”.

Predefined character classes #

\d : Matches any digit (0-9).
\D : Matches any non-digit character.
\s : Matches any whitespace character (space, tab, newline, etc.).
\S : Matches any non-whitespace character.
\w : Matches any word character (alphanumeric, including underscore).
\W : Matches any non-word character.

^(\d{3}-\d{2}-\d{4})|(\d{9})$ » » valid Social Security numbers