Characters and Strings

Last modified Tuesday, 23-Sep-2025 12:54:01 UTC.

Representation
- Characters are represented using ASCII encoding, using CHAR_BIT bits, as defined in limits.h.
- Strings are arrays of characters, with a null character '\0' after the last character of the string. (Not to be confused with the NULL pointer.)
- It is important to preserve the null terminating character as it is how C defines and manages variable length strings. All the C standard library functions require this for successful operation.
- You should use the many useful string handling functions and not really need to get your hands dirty dismantling and assembling strings.
Character handling functions
- ctype.h contains useful functions to convert and test single characters.
- int isalnum(int c) - True if c is alphanumeric.
- int isalpha(int c) - True if c is a letter.
- int isascii(int c) - True if c is ASCII .
- int iscntrl(int c) - True if c is a control character.
- int isdigit(int c) - True if c is a decimal digit
- int isgraph(int c) - True if c is a graphical character.
- int islower(int c) - True if c is a lowercase letter
- int isprint(int c) - True if c is a printable character
- int ispunct(int c) - True if c is a punctuation character.
- int isspace(int c) - True if c is a space character.
- int isupper(int c) - True if c is an uppercase letter.
- int isxdigit(int c) - True if c is a hexadecimal digit
- int toascii(int c) - Convert c to ASCII .
- int tolower(int c) - Convert c to lowercase.
- int toupper(int c) - Convert c to uppercase.
- CountCharTypes.c - Program to identify character types.
String conversion
- All the string handling functions are prototyped in string.h
- int atoi(const char *str); - Converts a string to an int (beware - returns 0 if not an "integer")
- long atol(const char *str); - Converts a string to a long
- long strtol(const char *str, char **endptr, int base); - Converts a string to a long, using the base
- double strtod(const char *str,char **endptr); - Converts a string to double-precision number
- double atof(const char *str); - Converts a string to double-precision number
- ReadLongInBase.c - Program that converts numbers to base 10
String copying, concatenation, comparison
- char *strcpy(const char *string1,const char *string2) - Copy string2 to string1. Chaos if string1 is not large enough.
- char *strncpy(const char *string1,const char *string2,size_t n) - Copy first n characters of string2 to string1 .
- char *strcat(const char *string1,char *string2) - Append string2 onto string1. Chaos if string1 is not large enough.
- char *strncat(const char *string1,char *string2, size_t n) - Append n characters from string2 to string1.
- int strcmp(const char *string1,const char *string2) - Compare string1 and string2 to determine alphabetic order.
- int strncmp(const char *string1,char *string2, size_t n) - Compare first n characters of two strings.
- int strcasecmp(const char *s1, const char *s2) - Case insensitive version of strcmp().
- int strncasecmp(const char *s1, const char *s2, int n) - Case insensitive version of strncmp().
- int strlen(const char *string) - Determine the length of a string.
- SplitLines.c - Program that splits long input strings
String tokenizing
- char *strtok(char *s1,const char *s2) - Break s1 into a sequence of tokens, each of which is delimited by one or more characters from s2.
- WordTokens.c - Program that finds words in a sentence.
String searching
- char *strchr(const char *string,int c) - Finds first occurrence of c in string.
- char *strrchr(const char *string,int c) - Finds last occurrence of c in string.
- char *strstr(const char *s1,const char *s2) - Finds the first occurrence of s2 in s1.
- char *strpbrk(const char *s1,const char *s2) - Finds the first occurrence in s1 of any character from s2
- char *strrpbrk(const char *s1,const char *s2) - Finds the last character in s1 also in s2.
- size_t strspn(const char *s1,const char *s2) - Counts the number of characters at the begining of s1 that are in s2.
- size_t strcspn(const char *s1,const char *s2) - Counts the number of characters at the begining of s1 that are not in s2.
- MatchBrackets.c - Program that matches brackets in a string
Regular expressions
- Defined in regex.h, this allows search for regexp strings in a program. Must include sys/types.h before regex.h.
- Basic regular expressions
  - A regular expression specifies a set of character strings.
  - The following one-character REs match a single character:
    - An ordinary character (not one of those discussed below) is a one-character RE that matches itself.
    - A backslash followed by any special character is a one-character RE that matches the special character itself. The special characters are:
      - ., *, [, and \, except when they appear within []; see below.
      - ^ at the beginning of an entire RE or when it immediately follows the left of a pair of [].
      - $ is special at the end of an entire RE.
    - A . is a one-character RE that matches any character except new-line.
    - A non-empty string of characters enclosed in [] is a one-character RE that matches any one character in that string. If, however, the first character of the string is a ^, the one-character RE matches any character except new-line and the remaining characters in the string. The ^ has this special meaning only if it occurs first in the string. A - may be used to indicate a range of consecutive characters, e.g., [0-9] is equivalent to [0123456789]. The - loses this special meaning if it occurs first (after an initial ^, if any) or last in the string. A ] does not terminate such a string when it is the first character within it (after an initial ^, if any), e.g., []a-f] matches either a ] or one of the ASCII letters a through f inclusive. The four characters ., *, [, and \ stand for themselves within such a string of characters.
  - The following rules may be used to construct REs from one- character REs:
    - A one-character RE is a RE that matches whatever the one-character RE matches.
    - A one-character RE followed by an * is a RE that matches 0 or more occurrences of the one-character RE. If there is any choice, the longest leftmost string that permits a match is chosen.
    - A one-character RE followed by an + is a RE that matches 1 or more occurrences of the one-character RE. If there is any choice, the longest leftmost string that permits a match is chosen.
    - A one-character RE followed by \{m\}, \{m,\}, or \{m,n\} is a RE that matches a range of occurrences of the one-character RE. The values of m and n must be non-negative integers less than 256; \{m\} matches exactly m occurrences; \{m,\} matches at least m occurrences; \{m,n\} matches any number of occurrences between m and n inclusive. Whenever a choice exists, the RE matches as many occurrences as possible.
    - A RE enclosed between the character sequences $ and $ is a RE that matches whatever the unadorned RE matches.
    - The expression \n matches the same string of characters as was matched by an expression enclosed between $ and $ earlier in the same RE. Here n is a digit; the subexpression specified is that beginning with the nth occurrence of $ counting from the left. For example, the expression ^\(.*$\1$ matches a line consisting of two repeated appearances of the same string.
  - A RE may be constrained to match words.
    - \< constrains a RE to match the beginning of a string or to follow a character that is not a digit, underscore, or letter. The first character matching the RE must be a digit, underscore, or letter.
    - \> constrains a RE to match the end of a string or to precede a character that is not a digit, underscore, or letter.
  - An entire RE may be constrained to match only an initial segment or final segment of a line (or both).
    - A ^ at the beginning of an entire RE constrains that RE to match an initial segment of a line.
    - A $ at the end of an entire RE constrains that RE to match a final segment of a line.
    - The construction ^entire RE$ constrains the entire RE to match the entire line.
- int regcomp(regex_t *preg,const char *pattern,int cflags); - Compiles the pattern. The parameter preg is a pointer. It points to the place where the compiled regular expression will be placed. The parameter cflags determines the type of RE that will be recognized.
- int regexec(const regex_t *preg,const char *string,size_t nmatch,regmatch_t pmatch[],int eflags); - Returns zero if some substring of string matches the regular expression in *preg. If there are matches, the nmatch and pmatch[] parameters can be used to see where the matches are. If nmatch is 0 then pmatch[] is ignored and can be NULL. eflags allows some variations, or can be 0.
- Use regfree to release the memory used by the compiled regular expression.
- MyGrep.c - Program that checks for a regexp

Exercises

Write the function htoi which converts a character string of hexadecimal digits (including an optional 0x or 0X) into its equivalent integer value. The allowable digits are 0-9, a-f, and A-F.
Answer: HtoI.c
Write a program that checks that the brackets in a string are balanced, i.e., open and closed brackets match exactly.
Write a program that edits a file, replacing all strings that match a supplied RE with another supplied string.

Exam Style Questions

Any of the short exercises.
How are strings represented in C?

Point out the errors in this program, which prevent it from compiling, and even if it did compile, from running properly.

#include <stdio.h>
#include <string.h>

int main(int argv,char *argv[]) {

    char Buffer[128];
    char *Data;

    Buffer = "the cat sat on the mat";
    Data = &Buffer[5];
    Buffer = Data;
    strcpy(Data,Buffer);

    return(0);
}

Write a program that read an integer and converts it to its string representation, then reads a string that looks like an integer and converts it to an integer.
Write a regular expression that matches all of the following strings:
- aaaaaaaaaa
- abba
- aabbb
but none of these strings:
- bbbaaa
- bbbbb
Which of the following regular expressions match which of the strings?
Regular expressions:
- ^a.b.*
- a+b*
- a*b*
Strings:
- baaaa
- aabb
- cba
- bbbbb

What is the output from the following program?

#include <stdio.h>
#include <string.h>

int main(int argc,char *argv) {

    char Buffer[128],Data[128];
    char *Offset;

    strcpy(Buffer,"What excellent stuffing");
    Offset = strtok(Buffer," ");
    printf("%s\n",Buffer);
    Offset = strtok(NULL," ");
    strcpy(Data,Offset);
    printf("%s\n",Data+3);
    Offset = strtok(NULL," ");
    printf("%s\n",Offset);
    strncpy(Data,&Offset[3],3);
    printf("%s\n",Data);

    return(0);
}