Regular Expressions

Regex Fundamentals

1. Delimiters: Characters used to mark the beginning and end of a pattern.
2. Literal Characters: Characters without special meaning. Ex: a to z, 0 to 9
3. Meta-Characters: Characters with special meaning. Ex:   . ^ $ * + ? {} [] \ | ()
4. `.` is a special metacharacter and it matches any single character except newline characters (\n, \r).
5. `\` is a special metacharacter and it is used to escape special characters or introduce special sequences (\n, \t, etc.)
6. Flags/Modifiers

   • `g` (global): Searches for all occurrences of the pattern within the text, rather than just the first one.
   • `i` (case-insensitive): Ignores case differences when matching letters.
   • `m` (multi-line): Changes the behavior of ^ and $ anchors to match the beginning and end of each line within the text, rather than just the beginning and end of the entire text.
   • `s` (dotAll): Allows the dot (.) to match newline characters (\n), which it doesn't do by default.
   • `u` (unicode): Enables full Unicode support for the regex pattern.
   • `y` (sticky): Matches only from the last index where a previous match ended.
7. Character Classes

   • Defining sets of characters to match [abc].
   • Ranges within character classes [a-z] or [0-9].
   • Combined ranges within character classes [a-z0-9].
   • Negated character classes [^abc].
8. Quantifiers

   • `*` matches 0 or more occurrences.
   • `+` matches 1 or more occurrences.
   • `?` matches 0 or 1 occurrence.
   • `{n}` matches exactly `n` occurrences.
   • `{n,}` matches `n` or more occurrences.
   • `{n,m}` matches between `n` and `m` occurrences.
9. Anchors

   • `^` asserts the position at the start of the string.
   • `$` asserts the position at the end of the string.
   • `m` (multi-line flag) (not an anchor): Changes the behavior of ^ and $ anchors to match the beginning and end of each line within the text, rather than just the beginning and end of the entire text.
10. Predefined/Shorthand Character Classes

    • `\d` matches any digit (equivalent to [0-9]).
    • `\D` matches any non-digit.
    • `\w` matches any word character (equivalent to [A-Za-z0-9_]).
    • `\W` matches any non-word character.
    • `\s` matches any whitespace character (spaces, tabs, line breaks).
    • `\S` matches any non-whitespace character.
11. Alternation

    • Using the vertical bar `|` to specify alternatives
12. Groups and Capturing

    • Grouping with parentheses `()`.
    • Capturing groups and backreferences `(\1\2)`
    • Non-capturing groups `(?:...)`.
    • Named capture groups (?<name>... )
    • Backreferences using names of named capture groups. \k<name>
13. Word Boundaries

    • Word boundaries `\b`
    • Non-word boundaries `\B`
14. Lookaheads and Lookbehinds (Lookarounds)

    • Positive lookahead `pattern1(?=pattern2)`
    • Negative lookahead `pattern1(?!pattern2)`
    • Positive lookbehind `(?<=pattern1)pattern2`
    • Negative lookbehind `(?<!pattern1)pattern2`
15. Debugging Regular Expressions

    • The current input position (the current position)
    • Lookahead where it is before the main pattern `(?=pattern2)pattern1`
    • Lookbehind where it is after the main pattern `pattern1(?<=pattern1)`
16. Substitutions or Replacements

    • $& Inserts the matched substring.
    • $` Inserts the portion of the string that precedes the matched substring.
    • $' Inserts the portion of the string that follows the matched substring.
    • $n Inserts the nth (1-indexed) capturing group where n is a positive integer less than 100.
    • $<name> Inserts the named capturing group where Name is the group name.
    • $$ Inserts a "$".
17. Regex in JavaScript