Chapter 1

Data Representation Grammar

 

Introduction

This chapter describes the core GEDCOM data representation language.

The generic data representation language defined in this chapter may be used to represent any form of structured information, not just genealogical data, using a sequential stream of characters.

Concepts

A GEDCOM transmission represents a database in the form of a sequential stream of related records. A record is represented as a sequence of tagged, variable-length lines, arranged in a hierarchy. A line always contains a hierarchical level number, a tag, and an optional value. A line may also contain a cross-reference identifier or a pointer. The GEDCOM line is terminated by a carriage return, a line feed character, or any combination of these.

The tag in the GEDCOM line, taken in its hierarchial context, identifies the information contained in the line, in the same sense that a field-name identifies a field in a database record. This means that the data is self-defining. Tags allow a field to occur any number of times within a record, including zero times. They also allow the use of different or new fields to be included in the GEDCOM data without introducing incompatibility, because the receiving system will ignore data which it does not understand and process only the data that it does understand.

The hierarchical relationships are indicated by a level number. Subordinate lines have a higher level number. The hierarchy allows a line to have sub-lines, which in turn may have their own sub-lines, and so forth. A line and its sub-lines constitute a context or enclosure, that is, a cluster of information pertaining directly to the same thing. This hierarchical arrangement corresponds with the natural hierarchy found in most structured information.

A series of one or more lines constitutes a record. The beginning of a new record is indicated by a line whose level number is 0 (zero).

In addition to hierarchical relationships, GEDCOM defines the inter-record relationships that allow a record to be logically related to other records, without introducing redundancy. These relationships are represented by two additional, but optional, parts of a line: a cross-reference pointer and a cross-reference identifier. The cross-reference pointer "points at" a related record, which is identified by a required, matching unique cross-reference identifier. The cross-reference identifier is analogous to a primary key in relational database terminology.

Grammar

This chapter defines the grammar for the GEDCOM format. The grammar is a set of rules that specify the character sequences that are valid for creating the expression of the GEDCOM line. The character sequences are described in terms of various combinations of elements (variables and/or constants). Elements may be described in terms of a set of other elements, some of which are selected from a set of alternative elements. Each element in the definition is separated by a plus sign (+) signifying that both elements are required. When there is a choice of different elements that can be used, the set of alternatives are listed between opening and closing square brackets ([]), with each choice separated by a vertical bar ([alternative_1 | alternative_2]). The user can read the grammar components of the selected element by substituting any sub-elements until all sub-elements have been resolved.

A GEDCOM transmission consists of a sequence of logical records, each of which consists of a sequence of gedcom_lines, all contained in a sequential file or stream of characters. The following rules pertain to the gedcom_line:

Long values can be broken into shorter GEDCOM lines by using a subordinate CONC or CONT tag. The CONC tag assumes that the accompanying subordinate value is concatenated to the previous line value without saving the carriage return prior to the line terminator. The CONT assumes that the subordinate line value is concatenated to the previous line, saving the carriage return.

The beginning of a new logical record is designated by a line whose level number is 0 (zero).

Each new level number must be no higher than the previous line plus 1.

Logical GEDCOM record sizes should be constrained so that they will fit in a memory buffer of less than 32K. GEDCOM files with records sizes greater than 32K run the risk of not being able to be loaded in some programs. Use of pointers to records, particularly NOTE records, should ensure that this limit will be sufficient. The size of embedded multimedia records can be controlled through chaining MULTIMEDIA_RECORDS (see multimedia record format on p. *.)

Any length constraints are given in characters, not bytes. When wide characters (characters wider than 8 bits) are used, byte buffer lengths should be adjusted accordingly.

Level numbers must be between 1 to 99 and must not contain leading zeroes, for example, level one must be 1, not 01.

The cross-reference ID has a maximum of 22 characters, including the enclosing at signs (@), and it must be unique within the GEDCOM transmission.

Pointers to records imply that the record pointed to does actually exists within the transmission. Future pointer structures may allow pointing to records within a public accessible database as an alternative.

The length of the GEDCOM TAG is a maximum of 31 characters, with the first 15 characters being unique.

The total length of a GEDCOM line, including leading white space, level number, cross-reference number, tag, value, delimiters, and terminator, must not exceed 255 (wide) characters.

Leading white space (tabs, spaces, and extra line terminators) preceding a GEDCOM line should be ignored by the reading system. Systems generating GEDCOM should not place any white space in front of the GEDCOM line.

 

Grammar Syntax

A gedcom_line has the following syntax:

gedcom_line:=

level + delim + [xref_id + delim +] tag + [delim + line_value +] terminator

level + delim + optional_xref_id + tag + delim + optional_line_value + terminator

for example:

1 NAME Will /Rogers/

The components used in the pattern above are defined below in alphabetical order. Some of the components are defined in terms of other primitive patterns. The spaces used in the patterns below are only to set them apart and are not a part of the resulting pattern. Character constants are specified in the hex form (0x20) which is the ASCII hex value of a space character. Character constants that are separated by a (-) dash represent any character with in that range from the first constant shown to and including the second constant shown.

alpha:=

[(0x41)-(0x5A) | (0x61)-(0x7A) | (0x5F) ]

where:

(0x41)-(0x5A)=A to Z

(0x61)-(0x7A)=a to z

(0x5F)=(_) underscore

alphanum:=

[alpha | digit ]

any_char:=

[alpha | digit | otherchar | (0x23) | (0x20) | (0x40)+(0x40) ]

where:

(0x23)=#

(0x20)=space character

(0x40)+(0x40)=@@

delim:=

[(0x20) ]

where:

(0x20)=space_character

digit:=

[(0x30)-(0x39) ]

where:

(0x30)-(0x39) = One of the digits 0, 1,2,3,4,5,6,7,8,9

escape:=

[(0x40) + (0x23) + escape_text + (0x40) + non_at ]

where:

(0x40)=@

(0x23)=#

escape_text:=

[any_char | escape_text + any_char ]

The escape_text is coded to meet the rules of a particular GEDCOM form.

level:=

[digit | level + digit ]

(Do not use non-significant leading zeroes such as 02.)

line_item:=

[any_char | escape | line_item + any_char | line_item + escape]

line_value:=

[ pointer | line_item ]

non_at:=

[alpha | digit | otherchar | (0x23) | (0x20 ) ]

where:

(0x20)=space character

(0x23)=#

null:= nothing

optional_line_value:= line_value + delim

optional_xref_Id:= xref_Id + delim

otherchar:=

[(0x21)-(0x22) | (0x24)-(0x2F) | (0x3A)-(0x3F) | (0x5B)-(0x5E) | (0x60) | (0x7B)-(0x7E) | (0x80)-(0xFE)]

where, respectively:

(0x21)-(0x22)=! "

(0x24)-(0x2F)=$ % & ' ( ) * + , - . /

(0x3A)-(0x3F)=: ; < = > ?

(0x5B)-(0x5E)=[ \ ] ^

(0x60)=`

(0x7B)-(0x7E)={ | } ~

(0x80)-(0xFE)=ANSEL characters above 127

Any 8-bit ASCII character except control characters (0x000x1F), alphanum, space ( ), number sign (#), at sign (@), _ underscore, and the DEL character (0x7F).

pointer:=

[(0x40) + alphanum + pointer_string + (0x40) ]

where:

(0x40)=@

pointer_char:=

[non_at ]

pointer_string:=

[null | pointer_char | pointer_string + pointer_char ]

tag:=

[alphanum | tag + alphanum ]

terminator:=

[carriage_return | line_feed | carriage_return + line_feed |

line_feed + carriage_return ]

xref_id:=

[pointer]

Description of Grammar Components

alpha:=

The alpha characters include the underscore, which is used to link word pieces together in forming tag names or tag labels.

any_char:=

Any 8-bit ASCII character except the control characters found in the range of 0x000x1F and 0x7F. If an @ is desired as part of the line_value, it must be written in GEDCOM as a double @, i.e., "3 doz. @ $20.00" must be stored as "3 doz. @@ $20.00."

delim:=

The delim (delimiter), a single space character, terminates both the variable-length level number and the variable-length tag. Note that space characters may also be present in a value.

escape:=

The escape is a character sequence in the grammar used to specify special processing, such as for switching character sets or for indicating an inclusion of a non-GEDCOM data form into the GEDCOM structure. The form of the escape sequence is:

@+#+escape_text+@+non_at.

Receiving systems should discard any space character which follows the escape sequences closing at-sign (@). If the character following the escape sequence's closing at-sign (@) is not a space character then it should be kept as a part of the text following the escape. Systems writing escape sequences should always output a space character following the escape sequence.

The specific format of the escape sequence is defined for the specific GEDCOM form being defined.

escape_text:=

The escape_text is defined to meet the requirements of a particular GEDCOM form.

level:=

The level number works the same way as the level of indentation in an indented outline, where indented lines provide detail about the item under which they are indented. A line at any level L is enclosed by and pertains directly to the nearest preceding line at level L-1. The Level L may increase by 1 at most. Level numbers must not contain leading zeroes, for example level one must be (1), not (01).

The enclosed subordinate lines at level L are said to be in the context of the enclosing superior line at level L-1. The interpretation of a tag must be in the context of the tags of the enclosing line(s) rather than just the tag by itself. Take the following record about an individual's birth and death dates, for example:

0 INDI

1 BIRT

2 DATE 12 MAY 1920

1 DEAT

2 DATE 1960

In this example, the expression DATE 12 MAY 1920 is interpreted within the INDI (individual) BIRT (birth) context, representing the individual's birth date. The second DATE is in the INDI.DEAT (individual's death) context. The complete meaning of DATE depends on the context.

Note:The above example is indented according to the level numbers to make the concept more obvious. In the actual GEDCOM data, the level numbers are lined up vertically, meaning they are the first character(s) of the GEDCOM line.

Some systems output indented GEDCOM data for better readability by putting space or tab characters between the terminator and the level number of the next line to visibly show the hierarchy. Also, some people have suggested allowing extra blank lines to visibly separate physical records. GEDCOM files produced with these features are not to be used when transmitting GEDCOM to other systems.

line_value:=

The line_value identifies an object within the domain of possible values allowed in the context of the tag. The combination of the tag, the line_value, and the hierarchical context of the supporting gedcom_lines provides the understanding of the enclosed values. This domain is defined by a specific grammar for representing a given GEDCOM form. (See Chapter 2, starting on page * for Lineage-Linked GEDCOM Form grammar.)

Values whose source information contains illegible parts of the value should be indicated by replacing the illegible part with an ellipsis (...).

Values are generally not encoded in binary or other abbreviation schemes for reducing space requirements, and they are generally constrained to be understandable by a typical user without decoding. This is intended to reduce the decoding burden on the receiving software. A GEDCOM-optimized data compression standard will be defined in the future to reduce space requirements. Meanwhile, users may agree to compress and decompress GEDCOM files using any compression system available to both sender and receiver.

The line_value within the context of a tag hierarchy of gedcom_lines represents one piece of information and corresponds to one field in traditional database or file terminology.

otherchar:=

Any 8-bit ASCII character except control characters (0x000x1F), alphanum, space ( ), number-sign (#), the at sign (@), and the DEL character (0x7F).

pointer:=

A pointer stands in the place of the context identified by the matching xref_id. Theoretically, a receiving system should be prepared to follow a pointer to find any needed value in a manner that is transparent to the logic of the subsystem that is looking for specific tags. This highly flexible facility will probably be used more in the future. For the time being, however, the use of pointers is explicitly defined within the GEDCOM form, such as the Lineage-Linked GEDCOM Form defined in Chapter 2 (see page *).

The pointer represents the association between two objects that usually reside in different records. Objects within a logical record can be associated. If this need exists, the pointer record composition contains an exclamation point (!) that separates the parent record's cross-reference ID from the specific substructure's cross-reference ID, which is at some subordinate level to the logical record at level zero. The cross-reference ID of the substructure subordinate to a zero level record, for inter-record associations is always composed of the Record ID number and the Substructure ID number, such as @I132!1@. Including the Record ID number in the pointer that associates objects within a record will allow the GEDCOM processors to build the index only at the record level and then search sequentially for the appropriate substructure cross-reference ID. The parent record ID is assumed when the cross-reference ID begins with a exclamation point (!) signifying an intra-record association.

Complex logical record structures are divided into small physical records to accommodate memory constraints, many-to-many relationships, and independent record creation and deletion.

The pointer must match a corresponding unique xref_id within the transmission, unless the colon (:) character is present (which will be used in the future as a network reference to a permanent file record). A pointer is given instead of duplicating an object, though the logical result is equivalent. An expanded traversal of a record tree includes following the pointer to related records to some depth, and splicing those records (logically) into the resultant expanded tree. Pointers may refer to either records which have not yet appeared in the transmission (forward reference) or to records that have already appeared earlier in the transmission (backward reference). This arrangement usually requires a preliminary pass to construct a look up table to support random access by xref_id during subsequent passes.

tag:=

A tag consists of a variable length sequence of alphanum characters. All user-defined tags, tags used that have not been defined in the GEDCOM standard, must begin with an underscore character (0x95).

The tag represents the meaning of the line_value within the context of the enclosing lines, and contributes to the meaning of enclosed subordinate lines. Specific tags are defined in Appendix A (starting on page *). The presence of a tag together with a value represents an assertion which the submitter wishes to communicate to a receiver. A tag with no value does not represent an assertion. If a tag is absent, no assertion is made, for example, no information is submitted. Information of a negative nature (such as knowing positively an event did not occur) is handled through the semantic definition of a tag and accompanying values that assert the information explicitly. It is not represented by absence of a tag.

Although formally defined tags are only three or four characters long, systems should prepare to handle user tags of greater length. Tags will be unique within the first 15 characters.

Valid combinations of specific tags, line_values, xref_ids, and pointers are constrained by the GEDCOM form defined for representing a given kind of information. (See Chapter 2, starting on page *, for the Lineage-Linked GEDCOM Form grammar.)

terminator:=

The terminator delimits the variable-length line_value and signals the end of the gedcom_line. The valid terminator characters are:

[carriage_return |

line_feed |

carriage_return line_feed |

line_feed carriage_return ]

xref_id:=

(See pointer, page *)

The xref_id is formed by any arbitrary combination of characters from the pointer_char set. The first character must be an alpha or a digit. The xref_id is not retained in the receiving system, and it may therefore be formed from any convenient combination of identifiers from the sending system. No meaning is attributed by the receiver to any part of the xref_id, other than its unique association with the associated record. The use of the colon (:) character is also reserved.

Examples:

The following are examples of valid but unrelated GEDCOM lines:

0 @1234@ INDI

. . .

1 AGE 13y

. . .

1 CHIL @1234@

. . .

1 NOTE This is a note field that is

2 CONT continued on the next line.

The first line has a level number 0, a xref_id of @1234@, an INDI tag, and no value.

The second line has a level number 1, no xref_id, an AGE tag, and a value of 13.

The third line has a level number 1, no xref_id, a CHIL tag, and a value of a pointer to a xref_id named @1234@.

Copyright © 1987, 1989, 1992, 1993, 1995 by The Church of Jesus Christ of Latter-day Saints. This document may be copied for purposes of review or programming of genealogical software, provided this notice is included. All other rights reserved.

Disclaimer: This HTML version of the GEDCOM 5.5 specification should be equivalent to the LDS wordperfect original. In the conversion process I have tried not to break anything however, the LDS original should always be considered the definitive version.
Clive Stubbings, October 2000