---
name: "regex.h"
section: "0p"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/regex.h"
---

# regex.h(0p)

regular expression matching types

## PROLOG

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

## NAME

regex.h — regular expression matching types

## SYNOPSIS

```
#include <regex.h>
```

## DESCRIPTION

The *\<regex.h\>* header shall define the structures and symbolic constants used by the *regcomp*(), *regexec*(), *regerror*(), and *regfree*() functions.

The *\<regex.h\>* header shall define the **regex\_t** structure type, which shall include at least the following member:

```
size_t    re_nsub    Number of parenthesized subexpressions.
```

The *\<regex.h\>* header shall define the **size\_t** type as described in *\<sys/types.h\>*.

The *\<regex.h\>* header shall define the **regoff\_t** type as a signed integer type that can hold the largest value that can be stored in either a **ptrdiff\_t** type or a **ssize\_t** type.

The *\<regex.h\>* header shall define the **regmatch\_t** structure type, which shall include at least the following members:

```
regoff_t    rm_so    Byte offset from start of string
                     to start of substring.
regoff_t    rm_eo    Byte offset from start of string of the
                     first character after the end of substring.
```

The *\<regex.h\>* header shall define the following symbolic constants for the *cflags* parameter to the *regcomp*() function:

- REG\_EXTENDED

  Use Extended Regular Expressions.

- REG\_ICASE

  Ignore case in match.

- REG\_NOSUB

  Report only success or fail in *regexec*().

- REG\_NEWLINE

  Change the handling of \<newline\>.

The *\<regex.h\>* header shall define the following symbolic constants for the *eflags* parameter to the *regexec*() function:

- REG\_NOTBOL

  The \<circumflex\> character (**'^'**), when taken as a special character, does not match the beginning of *string*.

- REG\_NOTEOL

  The \<dollar-sign\> (**'$'**), when taken as a special character, does not match the end of *string*.

The *\<regex.h\>* header shall define the following symbolic constants as error return values:

- REG\_NOMATCH

  *regexec*() failed to match.

- REG\_BADPAT

  Invalid regular expression.

- REG\_ECOLLATE

  Invalid collating element referenced.

- REG\_ECTYPE

  Invalid character class type referenced.

- REG\_EESCAPE

  Trailing \<backslash\> character in pattern.

- REG\_ESUBREG

  Number in \\*digit* invalid or in error.

- REG\_EBRACK

  **"\[\]"** imbalance.

- REG\_EPAREN

  **"\\(\\)"** or **"()"** imbalance.

- REG\_EBRACE

  **"\\{\\}"** imbalance.

- REG\_BADBR

  Content of **"\\{\\}"** invalid: not a number, number too large, more than two numbers, first larger than second.

- REG\_ERANGE

  Invalid endpoint in range expression.

- REG\_ESPACE

  Out of memory.

- REG\_BADRPT

  **'?'**, **'\*'**, or **'+'** not preceded by valid regular expression.

The following shall be declared as functions and may also be defined as macros. Function prototypes shall be provided.

```
int    regcomp(regex_t *restrict, const char *restrict, int);
size_t regerror(int, const regex_t *restrict, char *restrict, size_t);
int    regexec(const regex_t *restrict, const char *restrict, size_t,
           regmatch_t [restrict], int);
void   regfree(regex_t *);
```

The implementation may define additional macros or constants using names beginning with REG\_.

*The following sections are informative.*

## APPLICATION USAGE

None.

## RATIONALE

None.

## FUTURE DIRECTIONS

None.

## SEE ALSO

***\<sys\_types.h\>***

The System Interfaces volume of POSIX.1‐2017, *regcomp()*

## COPYRIGHT

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting\_bugs.html .