Question

I’ll preface this question by mentioning that while I’m far from a regular expressions guru, they are not completely foreign to me. Building a regular expression to search for a pattern inside a particular string generally isn’t a problem for me, but I have a (maybe?) unique situation.

I have a set of values, say:

028938
DEF567987
390987.456
GHI345928.039

I want to match a certain set of strings, such as:

• Strings composed of exactly 6 digits
• Strings composed of exactly 6 digits, a decimal, followed by exactly 3 more digits

In the above examples, the first and third values should be matched.

I’m using the regular expressions:

[0-9]{6}
[0-9]{6}.[0-9]{3}

Unfortunately, since all the above examples contain the specified pattern, all values are matched. This is not my intention.

So my question, in a nutshell, is how to write a regular expression that matches a string exactly and completely, with no additional characters to the right or left of the matched pattern? Is there a term for this type of matching? (Google was no help.) TIA

Answer 1

use ^ and $ to match the start and end of your string

^[0-9]{6}$
^[0-9]{6}\.[0-9]{3}$

Reference: http://www.regular-expressions.info/anchors.html

Also, as noted by Mikael Svenson, you can use the word boundary \b if you are searching for this pattern in a larger chunk of text.

Reference: http://www.regular-expressions.info/wordboundaries.html

You could also write both those regexes in one shot

^\d{6}(\.\d{3})?$