regular expression to define a language

Question

Can some one help with defining a regex for language L while strings in L don't contain 101 as a substring? (if only 0s and 1s are allowed)

I already have (1|0)*[^(101)] but it does reject all the strings in the language.

Answer 1

Add this negative lookahead:

^(?!.*?101)[01]+$

in front of your regex.