Regular expressions in C examples

Question

I m after some simple examples and best practices of how to use regular expressions in ANSI C  man regex h does not provide that much help

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While the answer above is good  I recommend using PCRE2  This means you can literally use all the regex examples out there now and not have to translate from some ancient regex    I made an answer for this already  but I think it can help here too    Regex In C To Search For Credit Card Numbers     YOU MUST SPECIFY THE UNIT WIDTH BEFORE THE INCLUDE OF THE pcre h   define PCRE2 CODE UNIT WIDTH 8  include  lt stdio h gt   include  lt string h gt   include  lt pcre2 h gt   include  lt stdbool h gt   int main     bool Debug   true  bool Found   false  pcre2 code  re  PCRE2 SPTR pattern  PCRE2 SPTR subject  int errornumber  int i  int rc  PCRE2 SIZE erroroffset  PCRE2 SIZE  ovector  size t subject length  pcre2 match data  match data    char   RegexStr         D    5 1-5  0-9  2          -   0-9  4          -   0-9  4          -   0-9  4       D      char   source    5111 2222 3333 4444    pattern    PCRE2 SPTR RegexStr     lt  lt  lt  lt  lt  This is where you pass your REGEX  subject    PCRE2 SPTR source     lt  lt  lt  lt  lt  This is where you pass your bufer that will be checked   subject length   strlen  char   subject         re   pcre2 compile    pattern                   the pattern      PCRE2 ZERO TERMINATED     indicates pattern is zero-terminated      0                         default options       amp errornumber              for error number       amp erroroffset              for error offset      NULL                      use default compile context        Compilation failed  print the error message and exit     if  re    NULL        PCRE2 UCHAR buffer 256     pcre2 get error message errornumber  buffer  sizeof buffer      printf  PCRE2 compilation failed at offset  d   s n    int erroroffset buffer     return 1        match data   pcre2 match data create from pattern re  NULL    rc   pcre2 match    re    subject                  the subject string      subject length           the length of the subject      0                        start at offset 0 in the subject      0                        default options      match data               block for storing the result      NULL    if  rc  lt  0        switch rc            case PCRE2 ERROR NOMATCH    printf  No match n           pcre2 match data free match data       pcre2 code free re       Found   0      return Found          break             Handle other special cases if you like            default  printf  Matching error  d n   rc     break          pcre2 match data free match data        Release memory used for the match      pcre2 code free re     Found   0                    data and the compiled pattern       return Found        if  Debug   ovector   pcre2 get ovector pointer match data   printf  Match succeeded at offset  d n    int ovector 0     if  rc    0    printf  ovector was not big enough for all the captured substrings n      if  ovector 0   gt  ovector 1         printf    K was used in an assertion to set the match start after its end  n       From end to start the match was     s n    int  ovector 0  - ovector 1           char    subject   ovector 1       printf  Run abandoned n      pcre2 match data free match data     pcre2 code free re     return 0     for  i   0  i  lt  rc  i          PCRE2 SPTR substring start   subject   ovector 2 i     size t substring length   ovector 2 i 1  - ovector 2 i     printf   2d     s n   i   int substring length   char   substring start          else    if rc  gt  0       Found   true             pcre2 match data free match data   pcre2 code free re   return Found       Install PCRE using   wget https   ftp pcre org pub pcre pcre2-10 31 zip make  sudo make install  sudo ldconfig   Compile using    gcc foo c -lpcre2-8 -o foo   Check my answer for more details

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Regular expressions actually aren t part of ANSI C  It sounds like you might be talking about the POSIX regular expression library  which comes with most  all    nixes  Here s an example of using POSIX regexes in C  based on this     include  lt regex h gt          regex t regex  int reti  char msgbuf 100       Compile regular expression    reti   regcomp  amp regex    a   alnum      0   if  reti        fprintf stderr   Could not compile regex n        exit 1         Execute regular expression    reti   regexec  amp regex   abc   0  NULL  0   if   reti        puts  Match      else if  reti    REG NOMATCH        puts  No match      else       regerror reti   amp regex  msgbuf  sizeof msgbuf        fprintf stderr   Regex match failed   s n   msgbuf       exit 1         Free memory allocated to the pattern buffer by regcomp      regfree  amp regex     Alternatively  you may want to check out PCRE  a library for Perl-compatible regular expressions in C  The Perl syntax is pretty much that same syntax used in Java  Python  and a number of other languages  The POSIX syntax is the syntax used by grep  sed  vi  etc

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man regex h reports there is no manual entry for regex h  but man 3 regex  gives you a page explaining the POSIX functions for pattern matching  The same functions are described in The GNU C Library  Regular Expression Matching  which explains that the GNU C Library supports both the POSIX 2 interface and the interface the GNU C Library has had for many years   For example  for an hypothetical program that prints which of the strings passed as argument match the pattern passed as first argument  you could use code similar to the following one    include  lt errno h gt   include  lt regex h gt   include  lt stdio h gt   include  lt stdlib h gt   include  lt string h gt   void print regerror  int errcode  size t length  regex t  compiled    int main  int argc  char  argv        regex t regex    int result     if  argc  lt  3                 The number of passed arguments is lower than the number of          expected arguments        fputs   Missing command line arguments n   stderr         return EXIT FAILURE           result   regcomp   amp regex  argv 1   REG EXTENDED     if  result                 Any value different from 0 means it was not possible to           compile the regular expression  either for memory problems          or problems with the regular expression syntax        if  result    REG ESPACE          fprintf  stderr    s n   strerror ENOMEM          else         fputs   Syntax error in the regular expression passed as first argument n   stderr         return EXIT FAILURE                         for  int i   2  i  lt  argc  i                result   regexec   amp regex  argv i   0  NULL  0         if   result                      printf     s  matches the regular expression n   argv i                    else if  result    REG NOMATCH                      printf     s  doesn t the regular expression n   argv i                    else                        The function returned an error  print the string               describing it               Get the size of the buffer required for the error message            size t length   regerror  result   amp regex  NULL  0             print regerror  result  length   amp regex                    return EXIT FAILURE                        Free the memory allocated from regcomp         regfree   amp regex     return EXIT SUCCESS     void print regerror  int errcode  size t length  regex t  compiled      char buffer length      void  regerror  errcode  compiled  buffer  length     fprintf stderr   Regex match failed   s n   buffer       The last argument of regcomp   needs to be at least REG EXTENDED  or the functions will use basic regular expressions  which means that  for example  you would need to use a  3   instead of a 3  used from extended regular expressions  which is probably what you expect to use   POSIX 2 has also another function for wildcard matching  fnmatch    It doesn t allow to compile the regular expression  or get the substrings matching a sub-expression  but it is very specific for checking when a filename match a wildcard  e g  it uses the FNM PATHNAME flag

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This is an example of using REG EXTENDED  This regular expression     -    0-9           0-9      n     Allows you to catch decimal numbers in Spanish system and international       include  lt regex h gt   include  lt stdlib h gt   include  lt stdio h gt  regex t regex  int reti  char msgbuf 100    int main int argc  char const  argv          while 1           fgets  msgbuf  100  stdin            reti   regcomp  amp regex     -    0-9           0-9      n    REG EXTENDED           if  reti                fprintf stderr   Could not compile regex n                exit 1                         Execute regular expression            printf   s n   msgbuf           reti   regexec  amp regex  msgbuf  0  NULL  0           if   reti                puts  Match                      else if  reti    REG NOMATCH                puts  No match                      else               regerror reti   amp regex  msgbuf  sizeof msgbuf                fprintf stderr   Regex match failed   s n   msgbuf               exit 1                         Free memory allocated to the pattern buffer by regcomp              regfree  amp regex

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It s probably not what you want  but a tool like re2c can compile POSIX -ish  regular expressions to ANSI C   It s written as a replacement for lex  but this approach allows you to sacrifice flexibility and legibility for the last bit of speed  if you really need it

[c] Regular expressions in C: examples?

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