java - RegEx validation name with underscore -

java - RegEx validation name with underscore -

i had validation like, string can contains alphanumeric , _ in between. string can start , end _ string can _

i wrote regex

^(_)?([a-za-z0-9]+)?([a-za-z0-9])+?(_)?$

my unit testes

assertequals(false, utilityfunction.isalphanumeric("__")); assertequals(true, utilityfunction.isalphanumeric("a")); assertequals(true, utilityfunction.isalphanumeric("_a")); assertequals(true, utilityfunction.isalphanumeric("a_")); assertequals(true, utilityfunction.isalphanumeric("_a_")); assertequals(true, utilityfunction.isalphanumeric("_a_a")); assertequals(true, utilityfunction.isalphanumeric("_a_a")); assertequals(false, utilityfunction.isalphanumeric("_a__a")); assertequals(true, utilityfunction.isalphanumeric("_a_a_")); assertequals(true, utilityfunction.isalphanumeric("a_a"));

getting failed @ "_a_a"

you can utilize regex

^(?!.*?__)\w+$ regex demo

java regex