I am writing an interpreter for assembly using lex and yacc. The problem is that I need to parse a word that will strictly be at the end of the file. I've read that there is an anchor $, which can help. However it doesn't work as I expected. I've wrote this in my lex file:
ABC$ {printf("QWERTY\n");}
The input file is:
ABC
without spaces or any other invisible symbols. So I expect the outputput to be QWERTY, however what I get is:
ABC
which I guess means that the program couldn't parse it. Then I thought, that $ might be a regular symbol in lex, so I changed the input file into this:
ABC$
So, if $ isn't a special symbol, then it will be parsed as a normal symbol, and the output will be QWERTY. This doesn't happen, the output is:
ABC$
The question is whether $ in lex is a normal symbol or special one.
In (f)lex, $ matches zero characters followed by a newline character.
That's different from many regex libraries where $ will match at the end of input. So if your file does not have a newline at the end, as your question indicates (assuming you consider newline to be an invisible character), it won't be matched.
As #sepp2k suggests in a comment, the pattern also won't be matched if the input file happens to use Windows line endings (which consist of the sequence \r\n), unless the generated flex file was compiled for Windows. So if you created the file on Windows and run the flex-generated scanner in a Unix environment, the \r will also cause the pattern to fail to match. In that case, you can use (f)lex's trailing context operator:
ABC/\r?\n { puts("Matched ABC at the end of a line"); }
See the flex documentation for patterns for a full description of the trailing context operator. (Search for "trailing context" on that page; it's roughly halfway down.) $ is exactly equivalent to /\n.
That still won't match ABC at the very end of the file. Matching strings at the very end of the file is a bit tricky, but it can be done with two patterns if it's ok to recognise the string other than at the end of the file, triggering a different action:
ABC/. { /* Do nothing. This ABC is not at the end of a line or the file */ }
ABC { puts("ABC recognised at the end of a line"); }
That works because the first pattern will match as long as there is some non-newline character following ABC. (. matches any character other than a newline. See the above link for details.) If you also need to work with Windows line endings, you'll need to modify the trailing context in the first pattern.
Related
How to replace all occurrences of some character or char-sequence with some other character or char-sequence, before flex lexes it. For example I want B\65R to match identifier rule as it is equivalent to BAR in my grammar. So, essentially I want to turn a sequence of \dd into its equivalent ascii character and then lex it. (\65 -> A, \66 -> B, …).
I know, I can first search the entire file for a sequence of \dd and replace it with equivalent character and then feed it to flex. But I wonder if there exists a better way. Something like writing a rule that matches \dd and then replacing it with corresponding alternative in the input stream, so that, I don't have to parse entire file twice.
Several options...
Next, flex is going to read from a filter that
substitutes "\dd" by "chr(dd)" (untested).
You could run something along the lines of
YYIN = popen("perl -pe 's/\\(\d\d)/chr($1)/e' ", "r");
yylex()....
The flex info manual provides allows whitespace in regular expressions using the "x" modifier in the (?r-s:pattern) form. It specifically offers a simple example (without whitespace)
(?:foo) same as (foo)
but the following program fails to compile with the error "unrecognized rule":
BAD (?:foo)
%%
{BAD} {}
I cannot find any form of (? that is acceptable as a rule pattern. Is the manual in error, or do I misunderstand?
The example in your question does not seem to reflect the question itself, since it shows neither the use of whitespace nor a x flag. So I'm going to assume that the pattern which is failing for you is something like
BAD (?x:two | lines |
of | words)
%%
{BAD} { }
And, indeed, that will not work. Although you can use extended format in a pattern, you can only use it in a definition if it doesn't contain a newline. The definition terminates at the last non-whitespace character on the definition line.
Anyway, definitions are overused. You could write the above as
%%
(?x:two | lines |
of | words ) { }
Which saves anyone reading your code from having to search for a definition.
I do understand that you might want to use a very long pattern in a rule, which is awkward, particularly if you want to use it twice. Regardless of the issue with newlines, this tends to run into problems with Flex's definition length limit (2047 characters). My approach has been to break the very long pattern into a series of definitions, and then define another symbol which concatenates the pieces.
Before v2.6, Flex did not chop whitespace off the end of the definition line, which also leads to mysterious "unrecognized rule" errors. The manual seems to still reflect the v2.5 behaviour:
The definition is taken to begin at the first non-whitespace character following the name and continuing to the end of the line.
How can I use the BBedit grep option to replace LaTeX commands like
\textcolor{blue}{Some text}
by the contents of the second set of braces, so
Some text
?
The BBEdit Grep Tutorial gives a lot of information and good examples on using the grep option in BBEdit. What you are trying to achieve is actually a variation of one of the examples. The solution is to enter the following:
Find: \\textcolor\{blue\}\{([^\}]*)\}
Replace: \1
The relevant part is the "Find" section. The first part: \\textcolor\{blue\}\{ basically searches for the content \textcolor{blue}{. You need the \s to escape special characters.
Next, we have the cryptic sequence ([^\}]*): The (...) saves everything inside the parentheses into the variable \1, which you can use in the "Replace" section to insert the content. The [^\}]* consists of ^\} which means match all characters which are not ^ a closing brace \}. With [...]* we say, match any number of "not brace" characters. Overall, this expression makes the grep match all characters which are not closing braces, and saves them into \1.
Finally, the expression ends with a \}, i.e. a closing brace, which is the end of what we want to find.
The "Replace" only contains \1, which is everything inside the parentheses (...) in the "Find" field.
I am trying to use the regular expression (?r-s:pattern) as mentioned in the Flex manual.
Following code works only when i input small letter 'a' and not the caps 'A'
%%
[(?i:a)] { printf("color"); }
\n { printf("NEWLINE\n"); return EOL;}
. { printf("Mystery character %s\n", yytext); }
%%
OUTPUT
a
colorNEWLINE
A
Mystery character A
NEWLINE
Reverse is also true i.e. if i change the line (?i:a) to (?i:A) it only considers 'A' as valid input and not 'a'.
If I remove the square brackets i.e. [] it gives error as
"ex1.lex", line 2: unrecognized rule
If I enclose the "(?i:a)" then it compiles but after executing it always goes to last rule i.e. "Mystery character..."
Please let me know how to use it properly.
I guess I am late.. :) Anyway, which flex version are you using, I have version 2.5.35 installed and correctly recognizes above pattern. Perhaps you're using old version!!!
Now regarding the enclosing with [] brackets. It works because as per [] regex rule it will try to match any of individual (, ?, i, :, a or ). Thats why a gets recognized and not A (because it is not in the list).
The way I read the manual, the rule without the square brackets should perform the case-insensitive matching you're looking for--I can't explain why you get an error at compile time. But you can achieve the same behavior in one of two ways. One, you can enumerate the upper and lower case characters in the character class:
%%
[Aa] { printf("color"); }
%%
Two, you can specify the case-insensitive scanner option, either on the command line as -i or --case-insensitive or in your .l file:
%%
%option case-insensitive
[a] {printf("color"); }
%%
I am using TeXnicCenter to edit a LaTeX document.
I now want to remove a certain tag (say, emph{blabla}} which occurs multiple times in my document , but not tag's content (so in this example, I want to remove all emphasization).
What is the easiest way to do so?
May also be using another program easily available on Windows 7.
Edit: In response to regex suggestions, it is important that it can deal with nested tags.
Edit 2: I really want to remove the tag from the text file, not just disable it.
Using a regular expression do something like s/\\emph\{([^\}]*)\}/\1/g. If you are not familiar with regular expressions this says:
s -- replace
/ -- begin match section
\\emph\{ -- match \emph{
( -- begin capture
[^\}]* -- match any characters except (meaning up until) a close brace because:
[] a group of characters
^ means not or "everything except"
\} -- the close brace
and * means 0 or more times
) -- end capture, because this is the first (in this case only) capture, it is number 1
\} -- match end brace
/ -- begin replace section
\1 -- replace with captured section number 1
/ -- end regular expression, begin extra flags
g -- global flag, meaning do this every time the match is found not just the first time
This is with Perl syntax, as that is what I am familiar with. The following perl "one-liners" will accomplish two tasks
perl -pe 's/\\emph\{([^\}]*)\}/\1/g' filename will "test" printing the file to the command line
perl -pi -e 's/\\emph\{([^\}]*)\}/\1/g' filename will change the file in place.
Similar commands may be available in your editor, but if not this will (should) work.
Crowley should have added this as an answer, but I will do that for him, if you replace all \emph{ with { you should be able to do this without disturbing the other content. It will still be in braces, but unless you have done some odd stuff it shouldn't matter.
The regex would be a simple s/\\emph\{/\{/g but the search and replace in your editor will do that one too.
Edit: Sorry, used the wrong brace in the regex, fixed now.
\renewcommand{\emph}[1]{#1}
any reasonably advanced editor should let you do a search/replace using regular expressions, replacing emph{bla} by bla etc.