Thomson's Construction (cont.) ① To recognize an empty string 8 ·To recognize a symbol a in the alphabet∑ If N(r)and N(r2)are NFAs for regular expressions ri and r2 For regular expression rr2 N() NFA for r r2 N(I2) CS308 Compiler Theory 15Thomson’s Construction (cont.) • To recognize an empty string ε i f ε • To recognize an empty string ε • To reco gnize a s ymbol a in the al phabet Σ a gy p i f • If N(r1) and N(r 2) are NFAs for regular expressions r1 and r 2 • Fl i or regu lar express ion r | 1 r 2 N(r1) i f NFA for r1 | r 2 ε ε ε N(r2) ε CS308 Compiler Theory 15