an IPv4 address with a CIDR prefix
Matches an IPv4 address with a CIDR prefix. It is anchored at both ends, so the whole string has to match. There is 1 capture group.
Pattern
/^(\d{1,3}\.){3}\d{1,3}/\d{1,2}$/- Match scope
- the whole string
- Capture groups
- 1
- What it catches in the first example
- 10.0.0.0/8
Try it yourself
Type any text and the parts this expression catches are highlighted. It runs in your browser and is never sent anywhere.
10.0.0.0/8
1 match
Examples that match
- 10.0.0.0/8
- 192.168.0.0/24
Examples that do not match
- 10.0.0.0
- 10.0.0.0/
Other expressions in this group
^0x[0-9a-f]+$a hex number written with the 0x prefix^[0-9a-f]{64}$a sixty-four character hash^[0-9a-f]{24}$a twenty-four character document id^\d{5}$a five-digit postal code^[A-Z]{2}$a two-letter country code^01[016789]-?\d{3,4}-?\d{4}$a Korean mobile number^[가-힣]{2,5}$a Hangul name of two to five syllables^\d{5}(-\d{4})?$a US ZIP codeHow to read this
- With ^ and $ around it, the whole string must match; without them the match can sit anywhere. Always anchor an expression you use for validation.
- Round brackets ( ) capture what they wrap. When you only need grouping, write (?: ) so the numbering of later groups does not shift.
- Every expression here is checked against its examples. Even so, "looks like an email" and "is a real inbox" remain different questions.
- Nested open-ended repeats such as (a+)+ can blow up on certain inputs. Look for that shape before pasting someone else’s expression into your code.
Frequently asked questions
Q. What is the regex for an IPv4 address with a CIDR prefix?
^(\d{1,3}\.){3}\d{1,3}/\d{1,2}$.
Q. Which strings match?
Text like 10.0.0.0/8 matches; text like 10.0.0.0 does not.
Q. Does the whole string have to match?
Yes. It is wrapped in ^ and $, so nothing but a full match passes.
Q. How do I use what it caught?
There is one capture group, so you can read them from match[1] onwards.