_P_a_t_t_e_r_n _M_a_t_c_h_i_n_g _a_n_d _R_e_p_l_a_c_e_m_e_n_t
	
	     grep(pattern, x, ignore.case=FALSE, extended=TRUE, value=FALSE)
	     sub(pattern, replacement, x,
	             ignore.case=FALSE, extended=TRUE)
	     gsub(pattern, replacement, x,
	             ignore.case=FALSE, extended=TRUE)
	
	_A_r_g_u_m_e_n_t_s:
	
	     pattern : character string containing a regular expres-
	               sion to be matched in the vector of character
	               string vec.
	
	           x : a vector of character strings where matches
	               is sought.
	
	 ignore.case : if FALSE, the pattern matching is case sensi-
	               tive and if TRUE, case is ignored during
	               matching.
	
	    extended : if TRUE, extended regular expression matching
	               is used, and if FALSE basic regular expres-
	               sions are used.
	
	       value : if FALSE, a vector containing the indices of
	               the matches determined by grep is returned,
	               and if TRUE, a vector containing the matching
	               elements themselves is returned.
	
	 replacement : a replacement for matched pattern in sub and
	               gsub.
	
	_D_e_s_c_r_i_p_t_i_o_n:
	
	     grep searches for matches to pattern given as its first
	     argument within the the vector of character strings
	     given as its second argument.
	
	     sub and gsub perform replacement of matches determined
	     by regular expression matching.  The functions differ
	     only in that sub replaces only the first occurence of a
	     pattern whereas gsub replaces all occurences.
	
	     The regular expressions used are those specified by
	     POSIX 1003.2, either extended or basic, depending on
	     the value of the extended argument.
	
	_N_o_t_e:
	
	     This function is based on the regex regular expression
	     library written by Henry Spencer of the University of
	     Toronto.
	
	_S_e_e _A_l_s_o:
	
	     match, pmatch.
	
	_E_x_a_m_p_l_e_s:
	
	     grep("[a-z]", letters)