Regular Expressions 101

Comment: https://regex101.com/r/fP1oQ6/2

Definition Group(?(DEFINE)# (?<W>\s*) (?<V>\d+(?:\.\d+)?(?&W)) (?<S>[-+]?) (?<O>(?&W)[-+*\/](?&W)) (?<E>(?&S)(?:(?&V)|\((?&W)(?&E)(?&W)\))(?:(?&O)(?&E))?)# )

Comment:

Group W(?<W>\s*)

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

Group V(?<V>\d+(?:\.\d+)?(?&W))

\d+a digit, repeated at least once

Non-Capturing Group(?:\.\d+)?

?repeats the group contents below at most once:

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

\d+a digit, repeated at least once

(?&W)calls group W as a subroutine

Group S(?<S>[-+]?)

Character Class[-+]?

?matches at most one character from the set below:

-the literal - (45₁₀ / 55₈ / 2D₁₆)

+the literal + (43₁₀ / 53₈ / 2B₁₆)

Group O(?<O>(?&W)[-+*\/](?&W))

(?&W)calls group W as a subroutine

Character Class[-+*\/]

-the literal - (45₁₀ / 55₈ / 2D₁₆)

+the literal + (43₁₀ / 53₈ / 2B₁₆)

*the literal * (42₁₀ / 52₈ / 2A₁₆)

\/the literal / (47₁₀ / 57₈ / 2F₁₆)

(?&W)calls group W as a subroutine

Group E(?<E>(?&S)(?:(?&V)|\((?&W)(?&E)(?&W)\))(?:(?&O)(?&E))?)

(?&S)calls group S as a subroutine

Non-Capturing Group(?:(?&V)|\((?&W)(?&E)(?&W)\))

1st Alternative(?&V)

(?&V)calls group V as a subroutine

2nd Alternative\((?&W)(?&E)(?&W)\)

\(the literal ( (40₁₀ / 50₈ / 28₁₆)

(?&W)calls group W as a subroutine

(?&E)calls group E as a subroutine

(?&W)calls group W as a subroutine

\)the literal ) (41₁₀ / 51₈ / 29₁₆)

Non-Capturing Group(?:(?&O)(?&E))?

?repeats the group contents below at most once:

(?&O)calls group O as a subroutine

(?&E)calls group E as a subroutine

Comment:

Group 6((?&E))

(?&E)calls group E as a subroutine

Global pattern flags

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

g modifier: global. Finds all matches instead of stopping after the first

J modifier: Allow duplicate subpattern names

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Search reference

Regular Expression

# https://regex101.com/r/fP1oQ6/2
(?(DEFINE)#
(?<W>\s*)
(?<V>\d+(?:\.\d+)?(?&W))
(?<S>[-+]?)
(?<O>(?&W)[-+*\/](?&W))
(?<E>(?&S)(?:(?&V)|\((?&W)(?&E)(?&W)\))(?:(?&O)(?&E))?)#
)#
((?&E))

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