const regex = new RegExp('(\\bBEGIN\\b([\\w\\n]*?)\\bEND\\b\\n)', 'g')
const str = `a
b
BEGIN
c
d
e
END
f
g
h
i
j
BEGIN
k
l
m
n
o
p
q
END
r
s
t
u
v
BEGIN
w
x
y
END
z`;
const subst = ``;
// The substituted value will be contained in the result variable
const result = str.replace(regex, subst);
console.log('Substitution result: ', result);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for JavaScript, please visit: https://developer.mozilla.org/en/docs/Web/JavaScript/Guide/Regular_Expressions