Question

I want to write a regex pattern that starts with a class name and has dot between class names and ends with a class name
each class name,starts with a letter.it is like class import in java,

I wrote this pattern but it is too complex and slow for validating and I think it does not work properly

^([a-zA-Z]([0-9]|_|[a-zA-Z])*)(([a-zA-Z]([0-9]|_|[a-zA-Z])*)|\\.)*([a-zA-Z]([0-9]|_|[a-zA-Z])*)$

for example my input string should be like this: “com.casp.common.StringUtils”

Answer 1

Firstly, simplification:

[0-9]|_|[a-zA-Z]

can be combined as

[0-9_a-zA-Z]

and actually it has a shorthand representation

\w

so your regex can first be simplified to this equivalent form:

^([a-zA-Z](\w)*)(([a-zA-Z](\w)*)|\.)*([a-zA-Z](\w)*)$

Now, we could further simplify by removing some redundant capture groups, and turn on case-insensitive matching:

^[a-z]\w*([a-z]\w*|\.)*[a-z]\w*$

Of course this regex is incorrect, it matches a....b, for instance.

The problem is because the \. is an “alternative”, which should not be, as a dot is required between each component. So it should probably transformed to:

^[a-z]\w*([a-z]\w*\.)*[a-z]\w*$

but it is still incorrect because it won’t match a.b, because of the leading part. We need to remove it:

^([a-z]\w*\.)*[a-z]\w*$

and probably rearrange it such that the repeating part is at the end, not the start:

^[a-z]\w*(\.[a-z]\w*)*$