Regex syntax

This documentation page describe the regex syntax supported for grammar's parameter regex.

The ASR text to match is always in lower case.

Matching one character

.             any character except new line (includes new line with s flag)
\d            digit (\p{Nd})
\D            not digit
\pN           One-letter name Unicode character class
\p{Greek}     Unicode character class (general category or script)
\PN           Negated one-letter name Unicode character class
\P{Greek}     negated Unicode character class (general category or script)

Character classes

[xyz]         A character class matching either x, y or z (union).
[^xyz]        A character class matching any character except x, y and z.
[a-z]         A character class matching any character in range a-z.
[[:alpha:]]   ASCII character class ([A-Za-z])
[[:^alpha:]]  Negated ASCII character class ([^A-Za-z])
[x[^xyz]]     Nested/grouping character class (matching any character except y and z)
[a-y&&xyz]    Intersection (matching x or y)
[0-9&&[^4]]   Subtraction using intersection and negation (matching 0-9 except 4)
[0-9--4]      Direct subtraction (matching 0-9 except 4)
[a-g~~b-h]    Symmetric difference (matching `a` and `h` only)
[\[\]]        Escaping in character classes (matching [ or ])

Any named character class may appear inside a bracketed [...] character class. For example, [\p{Greek}[:digit:]] matches any Greek or ASCII digit. [\p{Greek}&&\pL] matches Greek letters.

Precedence in character classes, from most binding to least:

1. Ranges: a-cd == [a-c]d
2. Union: ab&&bc == [ab]&&[bc]
3. Intersection: ^a-z&&b == ^[a-z&&b]
4. Negation

Composites

xy    concatenation (x followed by y)
x|y   alternation (x or y, prefer x)

Repetitions

x*        zero or more of x (greedy)
x+        one or more of x (greedy)
x?        zero or one of x (greedy)
x*?       zero or more of x (ungreedy/lazy)
x+?       one or more of x (ungreedy/lazy)
x??       zero or one of x (ungreedy/lazy)
x{n,m}    at least n x and at most m x (greedy)
x{n,}     at least n x (greedy)
x{n}      exactly n x
x{n,m}?   at least n x and at most m x (ungreedy/lazy)
x{n,}?    at least n x (ungreedy/lazy)
x{n}?     exactly n x

Empty matches

$     the end of text

^ (beginning of text) and word boundaries are not supported.

Grouping

(exp)          numbered capture group (indexed by opening parenthesis)

Escape sequences

\*          literal *, works for any punctuation character: \.+*?()|[]{}^$
\123        octal character code (up to three digits) (when enabled)
\x7F        hex character code (exactly two digits)
\x{10FFFF}  any hex character code corresponding to a Unicode code point
\u007F      hex character code (exactly four digits)
\u{7F}      any hex character code corresponding to a Unicode code point
\U0000007F  hex character code (exactly eight digits)
\U{7F}      any hex character code corresponding to a Unicode code point

Perl character classes (Unicode friendly)

These classes are based on the definitions provided in UTS#18:

\d     digit (\p{Nd})
\D     not digit
\s     whitespace (\p{White_Space})
\S     not whitespace
\w     word character (\p{Alphabetic} + \p{M} + \d + \p{Pc} + \p{Join_Control})
\W     not word character

ASCII character classes

[[:alnum:]]    alphanumeric ([0-9A-Za-z])
[[:alpha:]]    alphabetic ([A-Za-z])
[[:ascii:]]    ASCII ([\x00-\x7F])
[[:blank:]]    blank ([\t ])
[[:cntrl:]]    control ([\x00-\x1F\x7F])
[[:digit:]]    digits ([0-9])
[[:lower:]]    lower case ([a-z])
[[:print:]]    printable ([ -~])
[[:punct:]]    punctuation ([!-/:-@\[-`{-~])
[[:space:]]    whitespace ([\t\n\v\f\r ])
[[:word:]]     word characters ([0-9A-Za-z_])
[[:xdigit:]]   hex digit ([0-9A-Fa-f])

A note about alternation

Be cautious when you declare two or more competing format to match.

Alternation (the | operator) may be less acurate than multiple grammars instead.

In doubt, prefer multi-grammar mode to alternation.