regex.hとloopのパフォーマンス比較 ref: http://qiita.com/items/b749303f88dd030b339c

file2.txt

$ time ./regexTime
real    0m0.005s
user    0m0.002s
sys 0m0.002s

file3.txt

$ time ./loopTime
real    0m0.005s
user    0m0.002s
sys 0m0.002s

loopTime.cc

int i,j;
int c, l = strlen(str);
for (i = j = 0; i < l; i++) {
    c = OPENSSL_VERSION_TEXT[i];
    if ('0' <= c && c <= '9') {
        for (j = i + 1; j < l; j++) {
            c = str[j];
            if (c == ' '){
                break;
            }
        }
        break;
    }
}
for( int k = i ; k < j - i + 2; k++ ){
    putchar(str[k]);
}
return 0;

regexTime.cc

regex_t preg;
size_t nmatch = 5;
regmatch_t pmatch[nmatch];
int ret = regcomp(&preg, "[[:digit:]]+.[[:digit:]].[[:digit:]]",
                  REG_EXTENDED | REG_NEWLINE);
if( ret ){
    cout << "regex compile failed" << endl;
}
cout << "String: " << str << endl;
if( regexec(&preg, str, nmatch, pmatch, 0) != 0 ){
    cout << "no match" << endl;
}
else{
    for( int i = 0 ; i < nmatch ; i++ ){
        for( int j = pmatch[i].rm_so ; j < pmatch[i].rm_eo ; j++ ){
            cout << str[j];
        }
    }
    cout << endl;
}