15. Regexps

make 15_Regexp subdirectory. All code should reside there.

0. Look at lecture and try

   • to teacher: select appropriate subset of examples

   • examples of searching/matching — with grep

   • examples of search/replace — with sed 's/regexp/replacement/'

     • convert them to sed -E

1. Search with fortune -l output

   • "You" and "you" (use grep)

   • Any prononus from "you, me, he, she" list (with egrep --color, you will need an alternative)

     • try to match exact word (not any word part)

       • hint1: "[^[:alpha:]]" is non-letter character

       • hint2: "(^|a)word" matches both word at the beginning of the line or aword everywhere in the line.

2. (optional if there's time) Search/replace practice TODO

Using C regex

This is mandatory part

taken from here

0. readlines.c acting as simple cat; using getline is mandatory

   • ./readlines file prints all lines of file

   • notice repeating line=NULL, len=0 getline() parameters to make this work

   • do not forget to free() each line

1. relines.c acting as simple grep

   • ./relines "regex" file shows only lines containing regex

   • see regcomp and regexec (with nmatch=0, pmatch=NULL parameters it just searches a regexp)

2. reshow.c that takes in account grouping operations in pattern (less then than #define MAXGR 10)

   • See more on regexec:

     • each pmatch[] element is start and end of substring matching certain regexp

     • pmatch[0] is for whole regexp

     • pmatch[1] is for 1st group

     • pmatch[2] is for 2nd group

     • …
   • Use this code:

        1   if(!regexec(&r, line, MAXGR, pm, 0)) {
        2           fputs(line, stdout);
        3           for(i=0; i<MAXGR && pm[i].rm_so>=0; i++)
        4                   printf("\t%d/%d\n", pm[i].rm_so, pm[i].rm_eo);
        5   }
        6   free(line);
     

   • Suppose file textfile is

     ab
     abba
     00aaaEEEbbb11

     ./reshow "\(aaa\).*\(bbb\)" textfile must show

        1 $ ./reshow "\(aa*\).*\(bb*\)" textfile
        2 ab
        3         0/2
        4         0/1
        5         1/2
        6 abba
        7         0/3
        8         0/1
        9         2/3
       10 00aaaEEEbbb11
       11         2/11
       12         2/5
       13         10/11
       14 
     

3. replone.c

   • ./replone regexp replacement file prints file lines with regexp→replacement substitution.

   • No grouping operator in regexp (phiew! 😅)

   • Only first match is replaced (phiew again!)

   • All possible errors must be treated accurately (i. e. check functions' return values)

H/W

Finish all tasks