1. A method of coding a structured document comprising items that take at least one string of characters to be coded by substituting a coding index from at least one indexing dictionary for said at least one string of characters to be coded, said at least one indexing dictionary comprising entries, each entry associating a respective coding index with a corresponding string of characters, the method further comprising: selecting at least one indexing dictionary from among a plurality of indexing dictionaries; identifying at least one dictionary entry in the selected indexing dictionary whose character string is linked to the string of characters to be coded, by matching between at least a continuous part of the string of characters of the dictionary entry or the string of characters to be coded and a continuous subpart of the string of characters; and coding the string of characters to be coded by reference, according to the matching, to the at least one identified dictionary entry, wherein, said coding by reference comprises coding an identifier of the selected indexing dictionary from amongst the plurality of indexing dictionaries, an item of information identifying said entry identified in the selected indexing dictionary, an item of information on positioning which indicates a first character of the identified entry string of characters that matches the part or subpart of the string of characters to be coded and an item of information on length of the matching part and subpart representing a number of characters of the matching part and subpart; wherein said indexing dictionaries are distinct from the structured document, wherein the item of information identifying said indexing dictionary selected, the item of information identifying said entry, the item of information on positioning, and the item of length information, are distinct information items, wherein the selected indexing dictionary comprises an entry corresponding to an already coded substring of said string of characters to be coded, so that a subsequent substring of said string of characters to be coded is coded with reference to said already coded substring, and wherein the positioning information is selected from a set comprising a first code when said part or subpart of the string of characters to be coded corresponds to a start of the entry identified, a second code when said part or subpart of the string of characters to be coded corresponds to an end of the entry identified, and an offset value of the position of said part or subpart of the string of characters to be coded in the entry identified, in other cases.