Regular Expressions 101

Pro

Definition Group(?(DEFINE) # Note that everything is atomic, JSON does not need backtracking if it's valid # and this prevents catastrophic backtracking (?<json>(?>\s*(?&object)\s*|\s*(?&array)\s*)) (?<object>(?>\{\s*(?>(?&pair)(?>\s*,\s*(?&pair))*)?\s*\})) (?<pair>(?>(?&STRING)\s*:\s*(?&value))) (?<array>(?>\[\s*(?>(?&value)(?>\s*,\s*(?&value))*)?\s*\])) (?<value>(?>true|false|null|(?&STRING)|(?&NUMBER)|(?&object)|(?&array))) (?<STRING>(?>"(?>\\(?>["\\\/bfnrt]|u[a-fA-F0-9]{4})|[^"\\\0-\x1F\x7F]+)*")) (?<NUMBER>(?>-?(?>0|[1-9][0-9]*)(?>\.[0-9]+)?(?>[eE][+-]?[0-9]+)?)) )

Comment: Note that everything is atomic, JSON does not need backtracking if it's valid

Comment: and this prevents catastrophic backtracking

Group json(?<json>(?>\s*(?&object)\s*|\s*(?&array)\s*))

Atomic Group(?>\s*(?&object)\s*|\s*(?&array)\s*)

1st Alternative\s*(?&object)\s*

\s*any whitespace character, repeated any number of times

(?&object)calls group object as a subroutine

\s*any whitespace character, repeated any number of times

2nd Alternative\s*(?&array)\s*

\s*any whitespace character, repeated any number of times

(?&array)calls group array as a subroutine

\s*any whitespace character, repeated any number of times

Group object(?<object>(?>\{\s*(?>(?&pair)(?>\s*,\s*(?&pair))*)?\s*\}))

Atomic Group(?>\{\s*(?>(?&pair)(?>\s*,\s*(?&pair))*)?\s*\})

\{the literal { (123₁₀ / 173₈ / 7B₁₆)

\s*any whitespace character, repeated any number of times

Atomic Group(?>(?&pair)(?>\s*,\s*(?&pair))*)?

?repeats the group contents below at most once:

(?&pair)calls group pair as a subroutine

Atomic Group(?>\s*,\s*(?&pair))*

*repeats the group contents below any number of times:

\s*any whitespace character, repeated any number of times

,the literal , (44₁₀ / 54₈ / 2C₁₆)

\s*any whitespace character, repeated any number of times

(?&pair)calls group pair as a subroutine

\s*any whitespace character, repeated any number of times

\}the literal } (125₁₀ / 175₈ / 7D₁₆)

Group pair(?<pair>(?>(?&STRING)\s*:\s*(?&value)))

Atomic Group(?>(?&STRING)\s*:\s*(?&value))

(?&STRING)calls group STRING as a subroutine

\s*any whitespace character, repeated any number of times

:the literal : (58₁₀ / 72₈ / 3A₁₆)

\s*any whitespace character, repeated any number of times

(?&value)calls group value as a subroutine

Group array(?<array>(?>\[\s*(?>(?&value)(?>\s*,\s*(?&value))*)?\s*\]))

Atomic Group(?>\[\s*(?>(?&value)(?>\s*,\s*(?&value))*)?\s*\])

\[the literal [ (91₁₀ / 133₈ / 5B₁₆)

\s*any whitespace character, repeated any number of times

Atomic Group(?>(?&value)(?>\s*,\s*(?&value))*)?

?repeats the group contents below at most once:

(?&value)calls group value as a subroutine

Atomic Group(?>\s*,\s*(?&value))*

*repeats the group contents below any number of times:

\s*any whitespace character, repeated any number of times

,the literal , (44₁₀ / 54₈ / 2C₁₆)

\s*any whitespace character, repeated any number of times

(?&value)calls group value as a subroutine

\s*any whitespace character, repeated any number of times

\]the literal ] (93₁₀ / 135₈ / 5D₁₆)

Group value(?<value>(?>true|false|null|(?&STRING)|(?&NUMBER)|(?&object)|(?&array)))

Atomic Group(?>true|false|null|(?&STRING)|(?&NUMBER)|(?&object)|(?&array))

1st Alternativetrue

truethe literal text true (case sensitive)

2nd Alternativefalse

falsethe literal text false (case sensitive)

3rd Alternativenull

nullthe literal text null (case sensitive)

4th Alternative(?&STRING)

(?&STRING)calls group STRING as a subroutine

5th Alternative(?&NUMBER)

(?&NUMBER)calls group NUMBER as a subroutine

6th Alternative(?&object)

(?&object)calls group object as a subroutine

7th Alternative(?&array)

(?&array)calls group array as a subroutine

Group STRING(?<STRING>(?>"(?>\\(?>["\\\/bfnrt]|u[a-fA-F0-9]{4})|[^"\\\0-\x1F\x7F]+)*"))

Atomic Group(?>"(?>\\(?>["\\\/bfnrt]|u[a-fA-F0-9]{4})|[^"\\\0-\x1F\x7F]+)*")

Group NUMBER(?<NUMBER>(?>-?(?>0|[1-9][0-9]*)(?>\.[0-9]+)?(?>[eE][+-]?[0-9]+)?))

\Astart of string

(?&json)calls group json as a subroutine

\zend of string

Global pattern flags

x modifier: extended. Ignores unescaped whitespace and comments outside character classes

x

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25 tokens

Regular Expression

(?(DEFINE)
# Note that everything is atomic, JSON does not need backtracking if it's valid
# and this prevents catastrophic backtracking
(?<json>(?>\s*(?&object)\s*|\s*(?&array)\s*))
(?<object>(?>\{\s*(?>(?&pair)(?>\s*,\s*(?&pair))*)?\s*\}))
(?<pair>(?>(?&STRING)\s*:\s*(?&value)))
(?<array>(?>\[\s*(?>(?&value)(?>\s*,\s*(?&value))*)?\s*\]))
(?<value>(?>true|false|null|(?&STRING)|(?&NUMBER)|(?&object)|(?&array)))
(?<STRING>(?>"(?>\\(?>["\\\/bfnrt]|u[a-fA-F0-9]{4})|[^"\\\0-\x1F\x7F]+)*"))
(?<NUMBER>(?>-?(?>0|[1-9][0-9]*)(?>\.[0-9]+)?(?>[eE][+-]?[0-9]+)?))
)
\A(?&json)\z

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