Why PCRE regex only capture 19 groups?-CodePudding

My Question:

My regex pattern is: (a)(b)(c)(d)(e)(f)(g)(h)(i)(j)(k)(l)(m)(n)(o)(p)(q)(r)(s)(t)(u)(v)(w)(x)(y)(z)

and My string is: abcdefghijklmnopqrstuvwxyz

the code's output is:

i_0:0 i_1:26 i_2:0 i_3:1 i_4:1 i_5:2 i_6:2 i_7:3 i_8:3 i_9:4 i_10:4 i_11:5 i_12:5 i_13:6 i_14:6 i_15:7 i_16:7 i_17:8 i_18:8 i_19:9 i_20:9 i_21:10 i_22:10 i_23:11 i_24:11 i_25:12 i_26:12 i_27:13 i_28:13 i_29:14 i_30:14 i_31:15 i_32:15 i_33:16 i_34:16 i_35:17 i_36:17 i_37:18 i_38:18 i_39:19 i_40:0 i_41:0 i_42:0 i_43:0 i_44:0 i_45:0 i_46:0 i_47:0 i_48:0 i_49:0 i_50:0 i_51:0 i_52:0 i_53:0 i_54:0 i_55:0 i_56:0 i_57:0 i_58:0 i_59:0

Question: Why PCRE regex only capture 19 groups?

My Code

#include <pcre.h>
#include <iostream>

pcre* _rex;
pcre_extra* _rexEx;

void CompileRexStr(const std::string& rex) {
    const char* errorinfo;
    int errpos = 0;
    _rex = NULL;
    _rexEx = NULL;

    _rex = pcre_compile(rex.c_str(), PCRE_UTF8, &errorinfo, &errpos, NULL);
    _rexEx = pcre_study(_rex, PCRE_STUDY_JIT_COMPILE, &errorinfo);
}

int main(){
    std::string rex = "(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)(k)(l)(m)(n)(o)(p)(q)(r)(s)(t)(u)(v)(w)(x)(y)(z)";
    CompileRexStr(rex);

    std::string str = "abcdefghijklmnopqrstuvwxyz";
    int result[60] = {0};
    int cur = 0;
    int pos = pcre_exec(_rex, _rexEx, str.c_str(), str.length(), cur, 0, result, 60);

    for(int i=0;i < 60; i  ) {
        std::cout << "i_" << i << ":" << result[i] << " ";
    }

    return 0;
}

CodePudding user response：

It returns 19 capture groups, because you provided space to return 20 matches, and one is used for whole matching string

Captured substrings are returned to the caller via a vector of integers whose address is passed in ovector. The number of elements in the vector is passed in ovecsize, which must be a non-negative number. Note: this argument is NOT the size of ovector in bytes.

The first two-thirds of the vector is used to pass back captured substrings, each substring using a pair of integers. The remaining third of the vector is used as workspace by pcre_exec() while matching capturing subpatterns, and is not available for passing back information. The number passed in ovecsize should always be a multiple of three. If it is not, it is rounded down.

Source: Manual for PCRE

If you have 26 capture groups, you need to pass a vector containing (26 1)×3 = 81 element at least.