正则表达式，用于检查正确的字段数量

Question

I have a certain file where the fields are seperated by a comma.

bla,20171206123901,bla,

I want to check if the correct amount of fields is in the line by regular expression where the last comma is optional. At this example it should be exactly 3. What is the correct regular expression for that?

I thought that maybe this one could work, but it is not working

(.*,){3}(,)[0,1]

because this one also matches lines which have too much fields.

Any help would be really appreciated

Thank you

Answer 1

Here is pattern which you can try applying to each line:

^[^,]*,[^,]*,[^,]*,?$

This assumes that if the optional third comma does appear, that it is the last thing on the line. Also, the pattern allows for empty columns. If this is not the case, then replace [^,]* with [^,]+ everywhere in the pattern.

Another way to write the above takes advantage of repeated columns:

^(?:[^,]*,){2}[^,]*,?$

Here, you may replace 2 with the number of desired columns minus one . So for 3 columns, you would use {2} in the pattern.

Demo

Answer 2

This one should work:

^(?:[^,]+,[^,]*){2},?$

and you can change the number (number of columns minus one.) if you need to add more columns.

Test it here.