Repetition modifier
Description
Example
?
Zero or one. Equivalent to
{0,1}
.
/[a-z]\d?/i
matches letter followed by an
optional digit.
*
Zero or more (sometimes
called a “Klene star” or
“Klene closure”).
/[a-z]\d*/i
matches a letter followed by an
optional number, possibly consisting of multiple
digits.
+
One or more.
/[a-z]\d+/i
matches a letter followed by a
required number, possibly containing multiple
digits.
The Period Metacharacter and Escaping
In regex, the period is a special character that means “match anything” (except new‐
lines). Very often, this catch-all metacharacter is used to consume parts of the input
that you don’t care about. Let’s consider an example where you’re looking for a single
five-digit zip code, and then you don’t care about anything else on the rest of the line:
const
input
=
"Address: 333 Main St., Anywhere, NY, 55532. Phone: 555-555-2525."
;
const
match
=
input
.
match
(
/\d{5}.*/
);
You might find yourself commonly matching a literal period, such as the periods in a
domain name or IP address. Likewise, you may often want to match things that are
regex metacharacters, such as asterisks and parentheses. To escape any special regex
character, simply prefix it with a backslash:
const
equation
=
"(2 + 3.5) * 7"
;
const
match
=
equation
.
match
(
/\(\d \+ \d\.\d\) \* \d/
);
Many readers may have experience with
filename globbing, or being
able to use *.txt to search for “any text files.” The * here is a “wild‐
card” metacharacter, meaning it matches anything. If this is famil‐
iar to you, the use of * in regexes may confuse you, because it
means something completely different, and cannot stand alone.
The period in a regex is more closely related to the * in filename
globbing, except that it only matches a single character instead of a
whole string.
A True Wildcard
Because the period matches any character except newlines, how do you match any
character including newlines? (This comes up more often than you might think.)
There are lots of ways to do this, but probably the most common is
[\s\S]
. This
246 | Chapter 17: Regular Expressions
