Regular Expression Syntax

Question 1

\d

Answer 1

Most engines: one digit from 0 to 9

Question 2

\d

Answer 2

Python 3: one Unicode digit in any script

Question 3

\w

Answer 3

Most engines: “word character”: ASCII letter, digit or underscore

Question 4

\s

Answer 4

Most engines: “whitespace character”: space, tab, newline, carriage return, vertical tab

Question 5

\D

Answer 5

One character that is not adigit as defined by your engine’s \d

Question 6

\W

Answer 6

One character that is not aword character as defined by your engine’s \w

Question 7

\S

Answer 7

One character that is not awhitespace character as defined by your engine’s \s

Question 8

{3}

Answer 8

Exactly three times

Question 9

{2,4}

Answer 9

Two to four times

Question 10

{3,}

Answer 10

Three or more times

Question 11

*

Answer 11

Zero or more times

Question 12

?

Answer 12

Once or none

Question 13

.

Answer 13

Any character except line break

Question 14

.

Answer 14

A period (special character: needs to be escaped by a )

Question 15

\

Answer 15

Escapes a special character

Question 16

\

Answer 16

Escapes a special character

Question 17

|

Answer 17

Alternation / OR operand

Question 18

( )

Answer 18

Capturing group

Question 19

\1

Answer 19

Contents of Group 1

Question 20

\2

Answer 20

Contents of Group 2

Question 21

(?: )

Answer 21

Non-capturing group

Question 22

\t

Answer 22

Tab

Question 23

\r

Answer 23

Carriage return character

Question 24

\n

Answer 24

Line feed character

Question 25

\r\n

Answer 25

Line separator on Windows

Question 26

+

Answer 26

The + (one or more) is “greedy”

Question 27

?

Answer 27

Makes quantifiers “lazy”

Question 28

*

Answer 28

The * (zero or more) is “greedy”

Question 29

?

Answer 29

Makes quantifiers “lazy”

Question 30

{2,4}

Answer 30

Two to four times, “greedy”

Question 31

?

Answer 31

Makes quantifiers “lazy”

Question 32

[ abc ]

Answer 32

One of the characters in the brackets

Question 33

-

Answer 33

Range indicator

Question 34

[x-y]

Answer 34

One of the characters in the range from x to y

Question 35

[^x]

Answer 35

One character that is not x

Question 36

[^x-y]

Answer 36

One of the characters not in the range from x to y

Question 37

[\d\D]

Answer 37

One character that is a digit or a non-digit

Question 38

[\x41]

Answer 38

Matches the character at hexadecimal position 41 in the ASCII table, i.e. A

Answer 39

Start of string or start of linedepending on multiline mode. (But when [^inside brackets], it means “not”)

Question 40

$

Answer 40

End of string or end of linedepending on multiline mode. Many engine-dependent subtleties.

Question 41

\A

Answer 41

Beginning of string (all major engines except JS)

Question 42

(?i)

Answer 42

Case-insensitive mode (except JavaScript)

Question 43

(?s)

Answer 43

DOTALL mode (except JS and Ruby). The dot (.) matches new line characters (\r\n). Also known as “single-line mode” because the dot treats the entire input as a single line

Question 44

(?m)

Answer 44

Multiline mode (except Ruby and JS) ^ and $ match at the beginning and end of every line

Question 45

(?x)

Answer 45

Free-Spacing Mode mode (except JavaScript). Also known as comment mode or whitespace mode

Question 46

(?P…)

Answer 46

Python named group named bar

Question 47

(?=…)

Answer 47

Positive lookahead  
(?=\d{10})\d{5}

Question 48

(?<=…)

Answer 48

Positive lookbehind

(?<=\d)cat

Question 49

(?!…)

Answer 49

Negative lookahead

(?!theatre)the\w+

Question 50

(?<!…)

Answer 50

Negative lookbehind

\w{3}(?<!mon)ster

Question 51

(?(A)X|Y)

Answer 51

If proposition A is true, then match pattern X; otherwise, match pattern Y.

Question 52

(?(1)foo|bar)

Answer 52

If Group 1 has been set, the engine must match the literal characters foo. If not, it must match the literal characters bar

Question 53

(?(foo)…|…)

Answer 53

Check whether the Group named foo has been set.

Question 54

(?(-1)X|Y)

Answer 54

If the nearest capture group to the left of this conditional has been set, match pattern X; otherwise, match pattern Y.

Question 55

Alternation behavior

Answer 55

An alternation is expected to quit as soon as one of the alternatives matches, not hold out for the longest match.

You can force it to continue by adding a condition after the alternation that can’t be met until the whole token has been consumed. The simplest option would be an anchor ($) or a word boundary (\b).