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carbonyl, cyclic acetal, ether bond, ester bond, sulfonic acid ester bond, cyano, amide bond, -0----C(=O)-S-, and -0----C(=O)-NH-. 

5. The resist composition of claim 1 wherein the base polymer further comprises recurring units of at least one type selected from recurring units having the formulae ( dl) to (d3): 

wherein RA is each independently hydrogen or methyl, 1 11 

Z is a single bond, phenylene group, ---O-Z -, ----C(=0)---0-Z 11 - or ----C(=0)-NH-Z 11 -, Z $^{11 }$is a C$_{1 }$-C$_{6 }$ aliphatic hydrocarbylene group or phe- 

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nylene group, which may contain a carbonyl moiety, ester bond, ether bond or hydroxyl moiety, 2 

Z is a single bond or ester bond, 3 31 

Z is a single bond, -Z -C(=0)---0-, -Z 31 -0- or -Z 31 ---0-C(=0)-, Z 31 is a C 1 -C$_{12 }$ saturated hydrocarbylene group which may contain a carbonyl moiety, ester bond, ether bond, iodine or bromine, 4 

Z is methylene, 2,2,2-trifluoro-l,1-ethanediyl or carbo› nyl, 5 

Z is a single bond, methylene, ethylene, phenylene, fluorinated phenylene, -O-Z 51 -, ----C(=0)-0- Z51-, or ----C(=0)-NH-Z 51 -, Z 51 is a C 1 -C$_{6 }$ aliphatic hydrocarbylene group, phenylene group, fluo› rinated phenylene group, or trifluoromethyl-substituted phenylene group, which may contain a carbonyl moi› ety, ester bond, ether bond or hydroxyl moiety, 

R $^{21 }$to R $^{28 }$are each independently a C$_{1}$-C$_{20 }$ hydrocarbyl group which may contain a heteroatom, any two of R 23 , R $^{24 }$and R $^{25 }$or any two of R 26 , R $^{27 }$and R $^{28 }$may bond together to form a ring with the sulfur atom to which they are attached, and 

M- is a non-nucleophilic counter ion. 

6. The resist composition of claim 1, further comprising an acid generator. 

7. The resist composition of claim 1, further comprising an organic solvent. 

8. The resist composition of claim 1, further comprising a quencher. 

9. The resist composition of claim 1, further comprising a surfactant. 

10. A process for forming a pattern comprising the steps of applying the positive resist composition of claim 1 onto a substrate to form a resist film thereon, exposing the resist film to high-energy radiation, and developing the exposed resist film in a developer. 

11. The process of claim 10 wherein the high-energy radiation is i-line, KrF excimer laser, ArF excimer laser, EB, or EUV of wavelength 3 to 15 nm. 

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