CODEWITHSUNDEEP
cregex
lcazarre
lcazarre

Reputation: 743

How to test that a string follows a given pattern in C?

I would like to check whether a string follows a certain pattern. I tried sscanf, but I do not get the desired result.

The pattern is simple: it consists of:

Pattern examples:

I tried the following, but it would not check for the column:

sscanf(string, "while %[a-z,_]s %[:]c", test, column);

Would you have any suggestion?

Upvotes: 3

Views: 2730

Answers (3)

Joseph Quinsey
Joseph Quinsey

Reputation: 9962

This test for your pattern seems to work:

   int n = 0;
   Bool ok = sscanf(string, "while%*[ ]%*[A-Za-z_] :%*1[\n]%n", &n) == 0 && 
      n && !string[n];

It is nice and short, but there are (at least) two flaws:

  • It is ugly
  • It allows arbitrary whitespace before the colon, not just spaces (e.g. tabs, newlines)

The only way to handle zero or more spaces in sscanf is to use it twice, once for one or more, and then again for zero. For example, this code:

   char tail[4] = "";
   Bool ok = (sscanf(string, "while%*[ ]%*[A-Za-z_]%*[ ]%3c", tail) == 1 || 
              sscanf(string, "while%*[ ]%*[A-Za-z_]%3c",      tail) == 1) && 
              !strcmp(tail, ":\n");

Upvotes: 1

Bill Lynch
Bill Lynch

Reputation: 81926

A regular expression seems like a reasonable tool here:

#include <assert.h>
#include <regex.h>
#include <stdio.h>
#include <stdlib.h>

int main() {
    const char *expression = "^while +([a-zA-Z_]+) *:\n$";
    const char *input = NULL;
    regex_t regex;
    int rc;

    size_t nmatch = 2;
    regmatch_t pmatch[2];

    rc = regcomp(&regex, expression, REG_EXTENDED);
    assert(rc == 0);

    input = "while condition_a:\n";
    rc = regexec(&regex, input, nmatch, pmatch, 0);
    if(rc == 0) {
        printf("Match: %.*s\n", (int)(pmatch[1].rm_eo - pmatch[1].rm_so), input + pmatch[1].rm_so);
    } else if (rc == REG_NOMATCH) {
        printf("No match\n");
    } else {
        char msgbuf[64];
        regerror(rc, &regex, msgbuf, sizeof(msgbuf));
        printf("Regex match failed: %s\n", msgbuf);
    }

    input = "while test_b :\n";
    rc = regexec(&regex, input, nmatch, pmatch, 0);
    if(rc == 0) {
        printf("Match: %.*s\n", (int)(pmatch[1].rm_eo - pmatch[1].rm_so), input + pmatch[1].rm_so);
    } else if (rc == REG_NOMATCH) {
        printf("No match\n");
    } else {
        char msgbuf[64];
        regerror(rc, &regex, msgbuf, sizeof(msgbuf));
        printf("Regex match failed: %s\n", msgbuf);
    }

    regfree(&regex);
}

This will output:

Match: condition_a
Match: test_b

Upvotes: 1

user529758
user529758

Reputation:

Seems pretty straightforward to implement. You don't need either the unintuitive and quirky scanf(), nor non-portable (and, frankly, horrible) regular expressions:

int isValid(const char *s)
{
    // the string "while" followed by
    if (memcmp(s, "while", 5))
        return 0;

    s += 5;

    // one or more spaces, followed by
    if (!isspace(*s))
        return 0;

    while (isspace(*++s))
        ;

    // a string made of alpha characters or the underscore character,
    // (I assumed zero or more)
    while (isalpha(*s) || *s == '_')
        s++;

    // followed by zero or more spaces
    while (isspace(*s))
        s++;

    // followed by a column (':'),
    if (*s++ != ':')
        return 0;

    // followed by the newline character ('\n')
    if (*s++ != '\n')
        return 0;

    // here should be the end
    return !*s;
}

Upvotes: 6

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