37.1 PEX Definitions

Parsing expressions can be defined using the following syntax:

(and e1 e2…)

A sequence of PEXs that must all be matched. The and form is optional and implicit.

(or e1 e2…)

Prioritized choices, meaning that, as in Elisp, the choices are tried in order, and the first successful match is used. Note that this is distinct from context-free grammars, in which selection between multiple matches is indeterminate.

(any)

Matches any single character, as the regexp “.”.

string

A literal string.

(char c)

A single character c, as an Elisp character literal.

(* e)

Zero or more instances of expression e, as the regexp ‘*’. Matching is always “greedy”.

(+ e)

One or more instances of expression e, as the regexp ‘+’. Matching is always “greedy”.

(opt e)

Zero or one instance of expression e, as the regexp ‘?’.

symbol

A symbol representing a previously-defined PEG rule.

(range ch1 ch2)

The character range between ch1 and ch2, as the regexp ‘[ch1-ch2]’.

[ch1-ch2 "+*" ?x]

A character set, which can include ranges, character literals, or strings of characters.

[ascii cntrl]

A list of named character classes.

(syntax-class name)

A single syntax class.

(funcall e args…)

Call PEX e (previously defined with define-peg-rule) with arguments args.

(null)

The empty string.

The following expressions are used as anchors or tests – they do not move point, but return a boolean value which can be used to constrain matches as a way of controlling the parsing process (see Writing PEG Rules).

(bob)

Beginning of buffer.

(eob)

End of buffer.

(bol)

Beginning of line.

(eol)

End of line.

(bow)

Beginning of word.

(eow)

End of word.

(bos)

Beginning of symbol.

(eos)

End of symbol.

(if e)

Returns non-nil if parsing PEX e from point succeeds (point is not moved).

(not e)

Returns non-nil if parsing PEX e from point fails (point is not moved).

(guard exp)

Treats the value of the Lisp expression exp as a boolean.

Character-class matching can refer to the classes named in peg-char-classes, equivalent to character classes in regular expressions (see (elisp)Character Classes)