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spipatjs

This is a native (re)implementation of SNOBOL4/SPITBOL pattern matching for ES6 (JavaScript) which is fully Unicode compatible.

In SNOBOL4, PATTERN is a fundamental data type that can be composed. Matching is non-greedy, backtracking is used to progressively match larger prefixes.

Tested with nodejs v13.5.0

NOTE!!

In the following "Unicode character" means a JavaScript string representing a single Unicode code point, which may contain either one, or two UTF-16 16-bit code units (ie; a surrogate pair, as is the case for all emoji!)

Position values in .start and .stop, values passed/stored by the cursor function to one-based positions within the subject string.

Primative Patterns

The following primative Patterns are available as const variables:

abort

Immediately aborts the entire pattern match, signalling failure. This is a specialized pattern element, which is useful in conjunction with some of the special pattern elements that have side effects.

arb

Matches any string. First it matches the null string, and then on a subsequent failure, matches one character, and then two characters, and so on. It only fails if the entire remaining string is matched.

bal

Matches a non-empty string that is parentheses balanced with respect to ordinary () characters. Examples of balanced strings are "ABC", "A((B)C)", and "A(B)C(D)E". Bal matches the shortest possible balanced string on the first attempt, and if there is a subsequent failure, attempts to extend the string.

fail

The null alternation. Matches no possible strings, so it always signals failure. This is a specialized pattern element, which is useful in conjunction with some of the special pattern elements that have side effects.

fence

Matches the null string at first, and then if a failure causes alternatives to be sought, aborts the match (like abort). Note that using fence at the start of a pattern has the same effect as matching in anchored mode.

rem

Matches from the current point to the last character in the string. This is a specialized pattern element, which is useful in conjunction with some of the special pattern elements that have side effects.

succeed

Repeatedly matches the null string (it is equivalent to the alternation null.or(null, .....)) pattern element, which is useful in conjunction with some of the special pattern elements that have side effects.

Pattern contruction functions

and(....)

Takes any number of Pattern, String, Var, or Function arguments and returns a new Pattern which is a concatenation of all of the arguments.

any(SSVF)

Takes a String, Set, Var, or Function (returning a String or Set). Matches a single character that is any one of the given characters. Fails if the current character is not one of the given set of characters.

arbno(P)

Where P is any pattern, matches any number of instances of the pattern, starting with zero occurrences. It is thus equivalent to

null = pat('')
null.or(P.and(null.or(P.and(null.or( ....)))))

The pattern P may contain any number of pattern elements including the use of alternation and concatenation.

breakp(SSVF)

Where SSF is a String, Set, Var, or Function returning a String or Set. Matches a string of zero or more characters up to but not including a break character that is one of the characters given in the argument. Can match the null string, but cannot match the last character in the string, since a break character is required to be present.

breakx(SSVF)

Where SSVF is a String, Set, Var, or Function. Behaves exactly like breakp(SSVF) when it first matches, but if a string is successfully matched, then a subsequent failure causes an attempt to extend the matched string.

call(F)

Takes a function to call (with no arguments) for side-effect only; Any return value is ignored. Equivalent to pat('').imm(F)

cursor(VF)

Sets Var or calls Function with a one-based integer cursor position, defined as the count of Unicode characters that have been matched so far (including any start point moves).

fencef(P)

Where P is a pattern, attempts to match the pattern P including trying all possible alternatives of P. If none of these alternatives succeeds, then the fencef pattern fails. If one alternative succeeds, then the pattern match proceeds, but on a subsequent failure, no attempt is made to search for alternative matches of P. The pattern P may contain any number of pattern elements including the use of alternation and concatenation.

len(NF)

Where N is a positive integer (or Function returning one), matches the given number of Unicode characters. For example, Len(10) matches any string that is exactly ten characters long.

notany(SSVF)

Where S is a String, Set, Var, or Function returning a String or Set matches a single character that is not one of the given characters. Fails if the current character is one of the given set of characters.

nspan(SSVF)

Where SSF is a String, yadda, yadda, matches a string of zero or more characters that is among the characters given in the string. Always matches the longest possible such string. Always succeeds, since it can match the null string.

or(...)

Takes any number of Pattern, String, Var, or Function arguments and returns a Pattern which attempts to match each in turn.

pat(SFV)

Takes a String to match, a Var object, or a function that returns a string or pattern (to match) or a boolean (which, if true matches the null string, and if false fails) and returns a Pattern.

pos(NF)

Where NF is a positive integer (or Function), matches the null string if exactly N Unicode characters have been matched so far, and otherwise fails.

rpos(NF)

Where NF is a positive integer (or Function yadda yadda), matches the null string matches the null string if exactly N Unicode characters remain to be matched, and otherwise fails.

rtab(NF)

Where NF is a positive integer (or Function...), matches Unicode characters from the current position until exactly N characters remain to be matched in the string. Fails if fewer than N unmatched characters remain in the string.

tab(NF)

Where NF is a positive integer (or Function), matches from the current position until exactly N Unicode characters have been matched in all. Fails if more than N characters have already been matched.

span(SSVF)

Where SSF is a String, Set, Var, or Function, matches a string of one or more characters that are among the characters given. Always matches the longest possible such string. Fails if the current character is not one of the given set of characters.

and and or functions

Functions and and or are available to construct patterns.

Pattern object

Pattern object methods:

amatch(SUBJECT[, DEBUG])

Attempts an "anchored" match against the String SUBJECT. If DEBUG is true, uses console.log to trace the match. Returns a Match object on success, or null on failure.

and(...)

Takes any number of Pattern, String, Var, or Function arguments and returns a new Pattern which is a concatenation of all of the arguments.

imm(VF)

Takes a Var, or a Function with one argument, and returns a new Pattern. Each time the original Pattern matches, the Var.set method or Function will be called with the matching string.

onmatch(VF)

Takes a Var, or a Function with one argument, and returns a new Pattern. If the entire Pattern match succeeds, the Var.set method or Function will be called with each matching string.

or(...)

Takes any number of Pattern, String, Var, or Function arguments and returns a Pattern which attempts to match each in turn.

umatch(SUBJECT[, DEBUG])

Attempts an "unanchored" match against the String SUBJECT. If DEBUG is true, uses console.log to trace the match. If the match fails, successive attempts are made to match the given pattern each character of the subject string until a match succeeds, or until all possibilities have failed. Returns a Match object on success, or null on failure.

Match object

Match object members

matched

The String matched by the Pattern.

start

The (one-based) index of the first matched Unicode character in subject.

stop

The (one-based) index of the last matched Unicode character in subject.

subject

Array of Unicode characters for the SUBJECT string supplied to Pattern.amatch or Pattern.umatch.

Match object methods

replace(REPL)

Returns the SUBJECT String with the matched string replaced by the REPL String.

slice(START,END)

Returns a substring of the SUBJECT string, based on cursor units (one-based, counting the number of Unicode characters in the string)

Other Functions

cset(S)

Takes a String and returns a Set representing the Unicode characters in the string.

reverse(S)

Takes a strings and returns a string with the Unicode characters in reverse order.

Recursive Patterns

A Function returning a Pattern can be used to create a recursive Pattern:

var P = pat('A').or(pat('B').and(() => P));

On the first attempt, this pattern attempts to match the string "A". If this fails, then the alternative matches a "B", followed by an attempt to match P again. This second attempt first attempts to match "A", and so on. The result is a pattern that will match a string of B's followed by a single A.

This particular example could simply be written as nspan('B').and('A') but the use of recursive patterns in the general case can construct complex patterns which could not otherwise be built.

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SNOBOL/SPITBOL pattern matching in JavaScript

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