C String strspn() Function

What You’ll Learn

The strspn() function measures how many characters at the start of a string belong to an accept set. It is the standard way to skip leading spaces, digits, or letters when parsing text in C.

Definition and Usage

strspn stands for string span. Starting at s[0], it counts consecutive characters that appear in accept. When it hits a character not in the set—or reaches '\0'—it returns the count.

For "Hello, World!" with an alphabet accept string, the result is 5 because 'H' … 'o' match and ',' does not.

📝 Syntax

Standard C declaration:

size_t strspn(const char *s, const char *accept);

Parameters

• s — null-terminated string to examine.
• accept — C-string listing characters that may appear in the initial segment.

Return Value

• Number of bytes in the initial segment of s consisting only of characters from accept.
• 0 if s[0] is not in accept (when s is non-empty).

Header

• #include <string.h>
• Use %zu with printf for size_t results.

Examples Gallery

Compile with gcc strspn.c -std=c11 -o strspn. Print span lengths with %zu.

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

int main(void) {
    const char *text = "Hello, World!";
    const char *letters =
        "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";

    size_t length = strspn(text, letters);

    printf("Alphabet prefix length: %zu\n", length);
    printf("Prefix: %.*s\n", (int)length, text);

    return 0;
}

Alphabet prefix length: 5
Prefix: Hello

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

int main(void) {
    const char *line = "   \t  CodeToFun";

    size_t pad = strspn(line, " \t");

    printf("Padding: %zu chars\n", pad);
    printf("Content starts at: [%s]\n", line + pad);

    return 0;
}

Padding: 6 chars
Content starts at: [CodeToFun]

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

int main(void) {
    const char *value = "12345px";

    size_t digits = strspn(value, "0123456789");

    printf("Digit count: %zu\n", digits);
    printf("Number part: %.*s\n", (int)digits, value);
    printf("Suffix:      %s\n", value + digits);

    return 0;
}

Digit count: 5
Number part: 12345
Suffix:      px

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

int main(void) {
    const char *s = "  hello world";

    size_t spaces = strspn(s, " \t");
    size_t word   = strcspn(s + spaces, " \t");

    printf("spaces skipped: %zu\n", spaces);
    printf("word length:    %zu\n", word);
    printf("word:           %.*s\n", (int)word, s + spaces);

    return 0;
}

spaces skipped: 2
word length:    5
word:           hello

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

int main(void) {
    const char *csv = "one,two,three";
    const char *p = csv;

    while (*p) {
        p += strspn(p, ",");
        if (!*p) break;

        size_t len = strcspn(p, ",");
        printf("[%.*s] ", (int)len, p);

        p += len;
    }
    printf("\n");

    return 0;
}

[one] [two] [three] 

🚀 Common Use Cases

• Left trim — skip spaces and tabs before content.
• Numeric prefixes — read leading digits before a suffix.
• Charset validation — verify an ID starts with allowed symbols.
• Lexer-style scans — consume runs of the same character class.
• Pair with strcspn — build small splitters and tokenizers.

📝 Notes

• Returns a length, not a pointer—use s + strspn(...) to advance.
• Empty accept yields 0 for non-empty s.
• Case-sensitive byte matching—include both cases in accept if needed.
• Does not modify s; both arguments are read-only scans.
• Complement function: strcspn counts until a reject character appears.

⚡ Optimization

Library implementations often build a lookup table from accept for O(1) per-character tests. For tiny accept sets in homework-sized strings, the standard function is plenty fast. Profile before hand-rolling bitmap tables.

💡 Best Practices