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

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

asked Apr 5 at 22:32

U. WindlU. Windl

1718

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  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
  Apr 5 at 22:36
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  expandafterdefcsname macro2endcsnamecommand output is valid syntax. You then must use it via csname macro2endcsname.
  
  – Steven B. Segletes
  Apr 5 at 22:36
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  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
  Apr 5 at 22:41
  

  
  
  
  

add a comment | 

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

asked Apr 5 at 22:32

U. WindlU. Windl

1718

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  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
  Apr 5 at 22:36
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  expandafterdefcsname macro2endcsnamecommand output is valid syntax. You then must use it via csname macro2endcsname.
  
  – Steven B. Segletes
  Apr 5 at 22:36
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  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
  Apr 5 at 22:41
  

  
  
  
  

add a comment | 

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

asked Apr 5 at 22:32

U. WindlU. Windl

1718

share|improve this question

asked Apr 5 at 22:32

U. WindlU. Windl

1718

share|improve this question

asked Apr 5 at 22:32

U. WindlU. Windl

1718

asked Apr 5 at 22:32

U. WindlU. Windl

1718

asked Apr 5 at 22:32

U. WindlU. Windl

1718

1 Answer
1

active

oldest

votes

8

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 begintabular accesses tabular but begintabular* 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

edited Apr 5 at 23:24

Phelype Oleinik

25k54690

answered Apr 5 at 23:19

David CarlisleDavid Carlisle

498k4111441893

add a comment | 

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8

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 begintabular accesses tabular but begintabular* 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

edited Apr 5 at 23:24

Phelype Oleinik

25k54690

answered Apr 5 at 23:19

David CarlisleDavid Carlisle

498k4111441893

add a comment | 

8

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 begintabular accesses tabular but begintabular* 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

edited Apr 5 at 23:24

Phelype Oleinik

25k54690

answered Apr 5 at 23:19

David CarlisleDavid Carlisle

498k4111441893

add a comment | 

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 begintabular accesses tabular but begintabular* 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

edited Apr 5 at 23:24

Phelype Oleinik

25k54690

answered Apr 5 at 23:19

David CarlisleDavid Carlisle

498k4111441893

share|improve this answer

edited Apr 5 at 23:24

Phelype Oleinik

25k54690

answered Apr 5 at 23:19

David CarlisleDavid Carlisle

498k4111441893

edited Apr 5 at 23:24

Phelype Oleinik

25k54690

edited Apr 5 at 23:24

Phelype Oleinik

25k54690

answered Apr 5 at 23:19

David CarlisleDavid Carlisle

498k4111441893

answered Apr 5 at 23:19

David CarlisleDavid Carlisle

498k4111441893