When using a regular expression for validation.... don't forget the caret^ and the $tick

Let's say, you want to make sure the user types in a 3 digit number.

Simple! Can't be simpler:

\d\d\d

Let's test if this suits the following:

"500" -- Yep!
"999" -- Yep!
"123" -- Yep!

All good so far. Let's check that it fails on the following:

"5" -- Yep!
"99" -- Yep!
"ABC" -- Yep!
"" -- Yep!

That's good too. I guess we're done, right?

Uh-oh! It looks like we forget the caret^ and the $tick!

Therefore the following would return a match!

"HEY I like the film 2001 a space odyssey!" -- Oops! matches our expression!

as would

"But 2010 was just junk right?" -- Oops! another match!

What if we use a caret "^" and a dollar sign "$" to anchor the expression to the start and end...

^\d\d\d$

Now we'll get the result we want!

So when using a regular expression for validation, don't forget to anchor them to the front and the back, by using... you guessed it...

the caret^ and the $tick.

(donkey image lifted from an article about Team Building)