Print line using egrep only

Question

Print lines consisting of a definition of a single C variable of type int, without initialization, optionally preceded by unsigned, and optionally followed by any single line // comment.

I am using

egrep "^int.*[a-zA-Z0-9].*[^=].*;|^ unsigned int.*[a-zA-Z0-9].*[^=].*;" myfile.txt

It prints

int x=9;
int dftr2;

while it does not print

int x;

and it should not print

int x=9;

Here is the content of myfile.txt

int x;
int x=9;
int dftr2;
unsigned int dftrxe2;
unsigned int w=1;

The use of egrep is compulsory.

Answer 1

If I take your question literally (and don’t limit myself to trying to build on your incomplete answer) I believe that the correct answer is:

egrep '^\s*(unsigned\s+)?int\s+[_A-Za-z][_A-Za-z0-9]*\s*;\s*(//.*)?$' myfile.txt

Step by step:

• ^ to anchor the search to the beginning of the line.
• \s* to allow any number of whitespace characters (space or tab) to occur at the beginning of the line, before the declaration.
• (unsigned\s+)? to allow “unsigned” to appear zero or one time.
  • If it is present, it is followed by one or more spaces — \s+.
• int\s+ matches the “int” keyword, also followed by one or more spaces.
• [_A-Za-z] — the first character of a C variable name must be a letter or an underscore (_).
• [_A-Za-z0-9]* — subsequent characters of a C variable name may be letters, underscores, or digits.  There may be any number of them (including zero, since we’re talking about what follows the first character).
  • (Some C compilers probably impose a maximum identifier length.  I don’t remember whether the Standard does, and I’m not going to bother looking it up.)
• \s* to allow any number of spaces …
• … before the ;.
• \s* to allow any number of spaces after the semicolon.
• (//.*)? — optionally allow a // comment, and
• $ to anchor the search to the end of the line.

If you want, you can

• replace \s with [[:space:]] or [SpaceTab],
  • If you’re typing this at the keyboard, you may need to type Ctrk+V and then Tab in order to get an actual tab character in the command line.  (Of course this won’t be an issue if you’re writing a script.)
• replace [_A-Za-z] with [_[:alpha:]], and/or
• replace [_A-Za-z0-9]* with [_[:alnum:]].

Counter-examples:

The

egrep '^(unsigned )?int [^=]*;' myfile.txt

command, shown in another answer, prints the following lines:

int a, b;               // Multiple variables declared.
int c; int d;           // Multiple “int” declarations.
int e; float f;         // Multiple declarations where only the first is an “int”.
int g[9];               // Array.
int *h;                 // Pointer.
int func();             // Function.
int 3D;                 // Illegal variable name.
int 42;                 // Not even an illegal variable name.

(which it should not print), and skips the following lines:

  int s;                // Space(s) at the beginning of the line.
int     t;              // Tab after “int”.
unsigned  int u;        // Multiple spaces after “unsigned”.
unsigned        int v;  // Tab after “unsigned”.

(which it should print).

Answer 2

This looks for lines starting with int or unsigned int for which the statement terminates with ; and does not contain =:

$ egrep '^(unsigned )?int [^=]*;' myfile.txt
int x;
int dftr2;
unsigned int dftrxe2;

Observe that this works with comments as well and correctly ignores any = in the comment. Consider this test file:

$ cat myfile2.txt
int y; // y=height
int z=1; // z is depth

This is the result:

$ egrep '^(unsigned )?int [^=]*;' myfile2.txt
int y; // y=height

Lastly, note that egrep is deprecated. grep -E is preferred:

grep -E '^(unsigned )?int [^=]*;' myfile.txt

Answer 3

Searching for int or unsigned int and then excluding anything found that include an = indicating something that is being initialized seems like it should do the job.

grep -E '^int|^unsigned int' myfile.txt | grep -Ev '='