The operators =~ and !~ check values against regular expressions.

The expression a =~ b is true if the value of a matches the regular expression b.
a !~ b is true if the value of a does not match the regular expression b.

Just like comparison operators (such as < and >=) can be used to define bounds, the regular expression operators may also be used to define a set of strings through their unary forms.

fooBar:  "foo bar" =~ "^[a-z ]{1,100}$"
bazQuux: "baz Quux" !~ "[A-Z]"

#lowercaseLength3: =~"^[a-z]{3}$"
#noNumbers:        !~"[0-9]"

foo:       "foo" & #lowercaseLength3
BAR:       "BAR!!!" & #lowercaseLength3
theAnswer: "42" & #noNumbers
$ cue eval -ic file.cue
fooBar:    true
bazQuux:   false
foo:       "foo"
BAR:       _|_ // BAR: invalid value "BAR!!!" (out of bound =~"^[a-z]{3}$")
theAnswer: _|_ // theAnswer: invalid value "42" (out of bound !~"[0-9]")

Further details of CUE’s regular expression support can be found in the regexp package documentation.
CUE supports the RE2 regular expression syntax.