want to match in a string; there’s no (sensible) way to get those usernames into a
regex literal. This is where the
RegExp
constructor comes in, because it constructs the
regex from a string—which can be constructed dynamically. Let’s consider this
example:
const
users
=
[
"mary"
,
"nick"
,
"arthur"
,
"sam"
,
"yvette"
];
const
text
=
"User @arthur started the backup and 15:15, "
+
"and @nick and @yvette restored it at 18:35."
;
const
userRegex
=
new
RegExp(
`@(?:
${
users
.
join
(
'|'
)
}
)\\b`
,
'g'
);
text
.
match
(
userRegex
);
// [ "@arthur", "@nick", "@yvette" ]
The equivalent literal regex in this example would be
/@(?:mary|nick|arthur|sam|
yvette)\b/g
, but we’ve managed to construct it dynamically. Note that we have to
use double backslashes before the
b
(word boundary metacharacter); the first back‐
slash is to escape the second backslash in the string.
Conclusion
While this chapter has touched on the major points of regexes, it only scratches the
surface of the techniques, examples, and complexities inherent in regexes. Becoming
proficient at regexes is about 20% understanding the theory, and 80% practice. Using
a robust regex tester (such as
) can be very helpful when
you’re starting out (and even when you’re experienced!). The most important thing
you can take away from this chapter is an understanding of how a regex engine con‐
sumes input; a lack of understanding in that area is the source of a lot of frustration.
256 | Chapter 17: Regular Expressions
