LaTeX: Why are digits allowed in environments, but forbidden in commands?












3















Finding out that I can have an environment named env2, but no command cmd2, I read Why are some characters not allowed in command sequences, but still I wonder:



Why are digits allowed in environments, but forbidden in commands? In my understanding digits delimit command names (inherited from plain TeX), but why doesn't that restriction apply to environments? Wouldn't it have been more consistent to forbid digits in environment names as well?






macros environments







share|improve this question























  • You actually can use numbers in commands. If you do expandafterdefcsname mycommandwithanumber123456endcsname{} you'll define a command mycommandwithanumber123456. The environments are created using the csname...endcsname pair, so it's fine.

    – Phelype Oleinik
    7 hours ago











  • expandafterdefcsname macro2endcsname{command output} is valid syntax. You then must use it via csname macro2endcsname.

    – Steven B. Segletes
    7 hours ago













  • So the answer is "There are not forbidden, but they have to be used in a special way. Still it's recommended not to use them."?

    – U. Windl
    7 hours ago
















3















Finding out that I can have an environment named env2, but no command cmd2, I read Why are some characters not allowed in command sequences, but still I wonder:



Why are digits allowed in environments, but forbidden in commands? In my understanding digits delimit command names (inherited from plain TeX), but why doesn't that restriction apply to environments? Wouldn't it have been more consistent to forbid digits in environment names as well?






macros environments







share|improve this question























  • You actually can use numbers in commands. If you do expandafterdefcsname mycommandwithanumber123456endcsname{} you'll define a command mycommandwithanumber123456. The environments are created using the csname...endcsname pair, so it's fine.

    – Phelype Oleinik
    7 hours ago











  • expandafterdefcsname macro2endcsname{command output} is valid syntax. You then must use it via csname macro2endcsname.

    – Steven B. Segletes
    7 hours ago













  • So the answer is "There are not forbidden, but they have to be used in a special way. Still it's recommended not to use them."?

    – U. Windl
    7 hours ago














3












3








3


1






Finding out that I can have an environment named env2, but no command cmd2, I read Why are some characters not allowed in command sequences, but still I wonder:



Why are digits allowed in environments, but forbidden in commands? In my understanding digits delimit command names (inherited from plain TeX), but why doesn't that restriction apply to environments? Wouldn't it have been more consistent to forbid digits in environment names as well?






macros environments







share|improve this question














Finding out that I can have an environment named env2, but no command cmd2, I read Why are some characters not allowed in command sequences, but still I wonder:



Why are digits allowed in environments, but forbidden in commands? In my understanding digits delimit command names (inherited from plain TeX), but why doesn't that restriction apply to environments? Wouldn't it have been more consistent to forbid digits in environment names as well?






macros environments




macros environments






share|improve this question













share|improve this question











share|improve this question




share|improve this question










asked 7 hours ago









U. WindlU. Windl

1697




1697













  • You actually can use numbers in commands. If you do expandafterdefcsname mycommandwithanumber123456endcsname{} you'll define a command mycommandwithanumber123456. The environments are created using the csname...endcsname pair, so it's fine.

    – Phelype Oleinik
    7 hours ago











  • expandafterdefcsname macro2endcsname{command output} is valid syntax. You then must use it via csname macro2endcsname.

    – Steven B. Segletes
    7 hours ago













  • So the answer is "There are not forbidden, but they have to be used in a special way. Still it's recommended not to use them."?

    – U. Windl
    7 hours ago



















  • You actually can use numbers in commands. If you do expandafterdefcsname mycommandwithanumber123456endcsname{} you'll define a command mycommandwithanumber123456. The environments are created using the csname...endcsname pair, so it's fine.

    – Phelype Oleinik
    7 hours ago











  • expandafterdefcsname macro2endcsname{command output} is valid syntax. You then must use it via csname macro2endcsname.

    – Steven B. Segletes
    7 hours ago













  • So the answer is "There are not forbidden, but they have to be used in a special way. Still it's recommended not to use them."?

    – U. Windl
    7 hours ago

















You actually can use numbers in commands. If you do expandafterdefcsname mycommandwithanumber123456endcsname{} you'll define a command mycommandwithanumber123456. The environments are created using the csname...endcsname pair, so it's fine.

– Phelype Oleinik
7 hours ago





You actually can use numbers in commands. If you do expandafterdefcsname mycommandwithanumber123456endcsname{} you'll define a command mycommandwithanumber123456. The environments are created using the csname...endcsname pair, so it's fine.

– Phelype Oleinik
7 hours ago













expandafterdefcsname macro2endcsname{command output} is valid syntax. You then must use it via csname macro2endcsname.

– Steven B. Segletes
7 hours ago







expandafterdefcsname macro2endcsname{command output} is valid syntax. You then must use it via csname macro2endcsname.

– Steven B. Segletes
7 hours ago















So the answer is "There are not forbidden, but they have to be used in a special way. Still it's recommended not to use them."?

– U. Windl
7 hours ago





So the answer is "There are not forbidden, but they have to be used in a special way. Still it's recommended not to use them."?

– U. Windl
7 hours ago










1 Answer
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The restriction is in the scanner, tex's "eyes" in the texbook terminology. Any sequence of characters may be used as a command name, however after a character of catcode 0 (normally ) then tex scans the next non-letter (character of catcode other than 11) or a contiguous sequence of letters, and tokenizes it as a single csname token.



This means that foo123 normally parses as the token foo followed by the three tokens 1, 2, 3. Unless you make digits catcode 11. Similarly section* is normally the token section followed by the token *.



environment names use do not require to parse the name via the escape character, (they use the csname primitive but that's an implementation detail so begin{tabular} accesses tabular but begin{tabular*} accesses the command with name tabular* (not the two tokens that would normally be generated by parsing tabular*).



So technically it would be quite hard to not allow digits (or other characters) in environment names, especially when latex was designed there was not the memory available to add that kind of character-by-character check.






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    1 Answer
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    active

    oldest

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    1 Answer
    1






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    7














    The restriction is in the scanner, tex's "eyes" in the texbook terminology. Any sequence of characters may be used as a command name, however after a character of catcode 0 (normally ) then tex scans the next non-letter (character of catcode other than 11) or a contiguous sequence of letters, and tokenizes it as a single csname token.



    This means that foo123 normally parses as the token foo followed by the three tokens 1, 2, 3. Unless you make digits catcode 11. Similarly section* is normally the token section followed by the token *.



    environment names use do not require to parse the name via the escape character, (they use the csname primitive but that's an implementation detail so begin{tabular} accesses tabular but begin{tabular*} accesses the command with name tabular* (not the two tokens that would normally be generated by parsing tabular*).



    So technically it would be quite hard to not allow digits (or other characters) in environment names, especially when latex was designed there was not the memory available to add that kind of character-by-character check.






    share|improve this answer






























      7














      The restriction is in the scanner, tex's "eyes" in the texbook terminology. Any sequence of characters may be used as a command name, however after a character of catcode 0 (normally ) then tex scans the next non-letter (character of catcode other than 11) or a contiguous sequence of letters, and tokenizes it as a single csname token.



      This means that foo123 normally parses as the token foo followed by the three tokens 1, 2, 3. Unless you make digits catcode 11. Similarly section* is normally the token section followed by the token *.



      environment names use do not require to parse the name via the escape character, (they use the csname primitive but that's an implementation detail so begin{tabular} accesses tabular but begin{tabular*} accesses the command with name tabular* (not the two tokens that would normally be generated by parsing tabular*).



      So technically it would be quite hard to not allow digits (or other characters) in environment names, especially when latex was designed there was not the memory available to add that kind of character-by-character check.






      share|improve this answer




























        7












        7








        7







        The restriction is in the scanner, tex's "eyes" in the texbook terminology. Any sequence of characters may be used as a command name, however after a character of catcode 0 (normally ) then tex scans the next non-letter (character of catcode other than 11) or a contiguous sequence of letters, and tokenizes it as a single csname token.



        This means that foo123 normally parses as the token foo followed by the three tokens 1, 2, 3. Unless you make digits catcode 11. Similarly section* is normally the token section followed by the token *.



        environment names use do not require to parse the name via the escape character, (they use the csname primitive but that's an implementation detail so begin{tabular} accesses tabular but begin{tabular*} accesses the command with name tabular* (not the two tokens that would normally be generated by parsing tabular*).



        So technically it would be quite hard to not allow digits (or other characters) in environment names, especially when latex was designed there was not the memory available to add that kind of character-by-character check.






        share|improve this answer















        The restriction is in the scanner, tex's "eyes" in the texbook terminology. Any sequence of characters may be used as a command name, however after a character of catcode 0 (normally ) then tex scans the next non-letter (character of catcode other than 11) or a contiguous sequence of letters, and tokenizes it as a single csname token.



        This means that foo123 normally parses as the token foo followed by the three tokens 1, 2, 3. Unless you make digits catcode 11. Similarly section* is normally the token section followed by the token *.



        environment names use do not require to parse the name via the escape character, (they use the csname primitive but that's an implementation detail so begin{tabular} accesses tabular but begin{tabular*} accesses the command with name tabular* (not the two tokens that would normally be generated by parsing tabular*).



        So technically it would be quite hard to not allow digits (or other characters) in environment names, especially when latex was designed there was not the memory available to add that kind of character-by-character check.







        share|improve this answer














        share|improve this answer



        share|improve this answer








        edited 6 hours ago









        Phelype Oleinik

        24.9k54690




        24.9k54690










        answered 6 hours ago









        David CarlisleDavid Carlisle

        497k4111441892




        497k4111441892






























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