Regular expressions·a letter that may or may not be there

a letter that may or may not be there

Matches a letter that may or may not be there. It is anchored at both ends, so the whole string has to match. There are no capture groups.

Pattern
/^colou?r$/
Match scope
the whole string
Capture groups
0
What it catches in the first example
color

Try it yourself

Type any text and the parts this expression catches are highlighted. It runs in your browser and is never sent anywhere.

color

1 match

Examples that match

  • color
  • colour

Examples that do not match

  • colouur
  • colr

Other expressions in this group

^ab*c$none or many^ab+c$once or more^\d{4}$exactly four digits^\d{2,4}$between two and four digits^\d{2,}$two digits or more<.+?>one tag, taken as short as possible^<.+>$one chunk, taken as long as possible^(?:cat|dog)$either one of two words

How 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 a letter that may or may not be there?

^colou?r$.

Q. Which strings match?

Text like color matches; text like colouur 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 are no capture groups, so only the whole match (match[0]) comes back. Wrap the part you need in brackets to capture it.