Beginner Regex Question

[Juan González]

How would I write a regular expression to only look for the 4?

I want it to confirm that the 4 is either a [4|5] it can ignore all the digits after it

2 0.4995

[Ron McLeod]

Juan González wrote:How would I write a regular expression to only look for the 4?

Do you just want to know if a 4 exists in the string?

[Juan González]

I want to know if the 4 exists in that exact position

for more context I have this:

2 0.4995
3 0.3239
4 0.2541
5 0.2049
6 0.1653
7 0.1467
8 0.1279
9 0.1153
10 0.1044

I want to check if the number after the . is a 4 or a 5

I apologies if this doesn't make any sense

[Stephan van Hulst]

0[.][45]\d*+

[Ron McLeod]

You can use a tool like the one at RegexPlant to experiment different expressions and see what the results would be with a variety of inputs.

In this example, I specified a expression which was a literal dot (which needed to be escaped because it has a special meaning) and the number four.



Edit: sorry - I didn't notice that you wanted .4 or .5 - obviously the example above would need to be altered slightly to match

[Juan González]

Thank you all for the help, I really appreciate it!

[Tim Holloway]

Well, it doesn't make sense as an isolated question, but we'll assume you have a practical need.

To do what you need, you need advanced regex features that can look at the match or lack thereof and its context. For example, if your regex needs to match a "\\.[45]", which is a Java source string pattern for decimal followed by 4 or 5, you can use the match API to find the substring that preceded the match, the substring that comprises the matched character sequence, and the substring that follows the matched character sequence.

You can do very powerful things like match and capture values from multiple sub-patterns in your match pattern as well, but that's more than you need at the moment.