Regex flashcards: 50 patterns and concepts

All 50 cards

1. 1
   What do ^ and $ match?
   The start and the end of the input (of each line with the m flag)
2. 2
   What does \b match?
   A word boundary: a position between a word character and a non-word character (or the edge)
3. 3
   \d, \w, \s
   A digit, a word character ([A-Za-z0-9_]), whitespace

   Uppercase versions (\D, \W, \S) match the opposite.
4. 4
   What does . match?
   Any character except a line break

   With the s (dotAll) flag it matches line breaks too.
5. 5
   What does [^abc] match?
   Any single character except a, b or c
6. 6
   Match one lowercase letter or a digit
   [a-z0-9]
7. 7
   How do you match a literal - inside a character class?
   Put it first or last, or escape it: [-a-z], [a-z-], [a-z\-]
8. 8
   *, +, ?
   Zero or more, one or more, zero or one
9. 9
   {3}, {2,}, {2,5}
   Exactly 3, at least 2, between 2 and 5 (inclusive)
10. 10
    Greedy vs lazy quantifiers
    Greedy (.*) matches as much as possible; lazy (.*?) as little as possible
11. 11
    What does <.+> match in <b>bold</b>, and how do you match just <b>?
    The whole string <b>bold</b> (greedy). Use <.+?> or <[^>]+>
12. 12
    Capturing group vs non-capturing group
    (abc) captures the match for later use; (?:abc) only groups
13. 13
    Named capture group
    (?<year>\d{4}), read as match.groups.year in JavaScript
14. 14
    What does \1 mean inside a pattern?
    A backreference: the same text the first group matched

    (\w)\1 matches doubled letters like ll or oo.
15. 15
    Backreference to a named group
    \k<name>, e.g. (?<q>['"]).*?\k<q> matches a quoted string
16. 16
    What does cat|dog match?
    Either cat or dog

    Alternation has the lowest precedence: ^cat|dog$ means (^cat)|(dog$).
17. 17
    Positive lookahead
    x(?=y) matches x only if y follows, without consuming y
18. 18
    Negative lookahead
    x(?!y) matches x only if y does not follow
19. 19
    Lookbehind
    (?<=\$)\d+ matches digits preceded by a dollar sign; (?<!…) is the negative form
20. 20
    Password check: at least one digit and 8+ characters, using lookahead
    ^(?=.*\d).{8,}$
21. 21
    The g flag
    Global: find all matches instead of stopping at the first
22. 22
    The i flag
    Case-insensitive matching
23. 23
    The m flag
    Multiline: ^ and $ match at line breaks, not only at the ends of the input
24. 24
    The s flag
    dotAll: . also matches line breaks
25. 25
    The u and v flags
    Unicode mode: correct handling of code points and \p{…} property escapes; v adds set operations
26. 26
    The y flag
    Sticky: the match must start exactly at lastIndex
27. 27
    Match any letter in any script
    \p{L} with the u flag, e.g. /\p{L}+/u
28. 28
    Which characters need escaping outside a class?
    . * + ? ^ $ { } ( ) | [ ] \ / (the slash only in a /…/ literal)
29. 29
    Escape user input for use in a RegExp
    js

    const escaped = input.replace(/[.*+?^${}()|[\]\\]/g, '\\$&')
    new RegExp(escaped)

    Newer runtimes also have RegExp.escape(input).
30. 30
    Trim leading and trailing whitespace
    ^\s+|\s+$ with the g flag, replaced by an empty string

    In JavaScript just use str.trim().
31. 31
    Match an ISO date like 2026-10-15
    ^\d{4}-\d{2}-\d{2}$

    This checks the shape only; it accepts 2026-13-40.
32. 32
    A pragmatic email check
    ^[^\s@]+@[^\s@]+\.[^\s@]+$

    Fully validating email by regex is impractical; send a confirmation link instead.
33. 33
    Collapse runs of whitespace into one space
    js

    text.replace(/\s+/g, ' ')

34. 34
    Get every match with its groups
    js

    for (const m of text.matchAll(/(?<key>\w+)=(?<value>\w+)/g)) {
      console.log(m.groups.key, m.groups.value)
    }

    matchAll requires the g flag.
35. 35
    Reorder a date with capture groups in replace
    js

    '2026-10-15'.replace(/(\d+)-(\d+)-(\d+)/, '$3/$2/$1') // '15/10/2026'

36. 36
    Use a named group in a replacement string
    $<name>, e.g. .replace(/(?<user>\w+)@/, '$<user> at ')
37. 37
    What does $& mean in a replacement?
    The whole matched text
38. 38
    test vs match vs exec
    test returns a boolean; str.match returns matches; exec returns one match with groups and advances lastIndex with g
39. 39
    Why does a g regex reused with test() alternate true and false?
    It keeps lastIndex between calls, so the next search starts after the previous match

    Drop the g flag for yes/no checks, or reset lastIndex = 0.
40. 40
    Split on commas with optional spaces
    js

    'a, b,c'.split(/\s*,\s*/) // ['a', 'b', 'c']

41. 41
    What is catastrophic backtracking?
    Exponential running time when nested quantifiers can match the same text in many ways, as in (a+)+$ against aaaa…b
42. 42
    How do you avoid catastrophic backtracking?
    Avoid nested quantifiers over overlapping text, make alternatives mutually exclusive, and prefer specific classes like [^"]* over .*
43. 43
    Atomic groups and possessive quantifiers
    (?>…) and a++ never give back what they matched, preventing backtracking

    Supported by PCRE, Java and .NET; JavaScript has no possessive quantifiers.
44. 44
    What does \B match?
    A position that is not a word boundary

    \Bcat\B finds cat inside concatenate but not the word cat.
45. 45
    Whole-word search for cat
    \bcat\b
46. 46
    Match a hex color like #1a2B3c
    ^#(?:[0-9a-fA-F]{3}){1,2}$

    Accepts the 3- and 6-digit forms.
47. 47
    Match an empty line
    ^$ with the m flag
48. 48
    POSIX character class for digits
    [[:digit:]]

    Used by grep, sed and awk; JavaScript doesn’t support POSIX classes.
49. 49
    grep: basic vs extended regex
    In basic (default) mode + ? | { } ( ) need backslashes; grep -E treats them as operators
50. 50
    Which regex flavors allow lookbehind of any length?
    JavaScript and .NET; Python’s re needs a fixed length and PCRE a bounded one