عبارت باقاعده
نحو (سینتکس) > عبارت باقاعده بر مبنای پازیکس

یونیکس‌های سنتی نیز همان قواعد مشترک را در عبارت با قاعده دارد اما امکان دارد در بعضی از ابزار متفاوت است. پازیکس (POSIX) که مخفف «‎ Portable Operating System Interface [for Unix]‎» است، عبارت است از مجموعه استانداردهایی که برای نامگذاری و تعریف شمایل رابط برنامه‌نویسی کاربردی در محیط‌های شبه-یونیکس در آی‌تریپل‌ایی تعریف شده‌اند. این استانداردها تحت نام کلی IEEE 1003 و نام بین‌المللی ISO/IEC 9945 شناخته می‌شوند، امکان همسان‌سازی و ارتباط و پرت کردن آسان‌تر بین محیط‌های یادشده را فراهم می‌آورد. واژهٔ پازیکس پیشنهاد بنیان‌گذار بنیاد نرم‌افزار آزاد، ریچارد استالمن بود.
در سینتکس BREکه مخفف (Basic Regular Expressions) اغلب کاراکترها تحت‌الفظی است و فقط با خودشان برابرند؛ مثلاً a برابر "a" است. استثناهای زیر نیز وجود دارد که متا کاراکتر  گویند.
| Metacharacter | Description |
| --- | --- |
.: Matches any single character (many applications exclude newlines, and exactly which characters are considered newlines is flavor-, character-encoding-, and platform-specific, but it is safe to assume that the line feed character is included). Within POSIX bracket expressions, the dot character matches a literal dot. For example, a.c matches "abc", etc. , but [a.c] matches only "a", ".", or "c".
[ ]: A bracket expression. Matches a single character that is contained within the brackets. For example, [abc] matches "a", "b", or "c". [a-z] specifies a range which matches any lowercase letter from "a" to "z". These forms can be mixed: [abcx-z] matches "a", "b", "c", "x", "y", or "z", as does [a-cx-z]. The - character is treated as a literal character if it is the last or the first (after the ^) character within the brackets: [abc-], [-abc]. Note that backslash escapes are not allowed. The ] character can be included in a bracket expression if it is the first (after the ^) character: []abc].
[^ ]: Matches a single character that is not contained within the brackets. For example, [^abc] matches any character other than "a", "b", or "c". [^a-z] matches any single character that is not a lowercase letter from "a" to "z". Likewise, literal characters and ranges can be mixed.
^: Matches the starting position within the string. In line-based tools, it matches the starting position of any line.
$: Matches the ending position of the string or the position just before a string-ending newline. In line-based tools, it matches the ending position of any line.
BRE: \( \) ERE: ( ): Defines a marked subexpression. The string matched within the parentheses can be recalled later (see the next entry, \n). A marked subexpression is also called a block or capturing group.
\n: Matches what the nth marked subexpression matched, where n is a digit from 1 to 9. This construct is theoretically irregular and was not adopted in the POSIX ERE syntax. Some tools allow referencing more than nine capturing groups.
*: Matches the preceding element zero or more times. For example, ab*c matches "ac", "abc", "abbbc", etc. [xyz]* matches "", "x", "y", "z", "zx", "zyx", "xyzzy", and so on. \(ab\)* matches "", "ab", "abab", "ababab", and so on.
BRE: \{m,n\} ERE: {m,n}: Matches the preceding element at least m and not more than n times. For example, a\{3,5\} matches only "aaa", "aaaa", and "aaaaa". This is not found in a few older instances of regular expressions.