This document describes several web scraping functions: 1. A web scraper function that extracts data from an HTML document as a character object based on CSS selectors. 2. A function to check if an object is a valid HTML element. 3. A tag counter function that counts the number of tags in a string. 4. Closing tag locator functions that find the closing tags for given opening tags. 5. A content extractor function that extracts HTML elements based on tag positions and can remove tags or construct a data frame. 6. A table constructor function that creates a data frame from an HTML table. 7. A content remover function that removes unwanted content from extractor

1. Web Scrapping Functions
1. Web scraper
Description
Extracts data from the internet as a HTML document object and stores it as an R character
object.
Usage
webScraper(url, css, ..., asis = TRUE, constructTable = FALSE, withOutTags = FALSE)
Arguments
url: Internet or file address to read a valid HTML document. css: A character vector with css
selectors to use for data extraction ...: Other arguments to be passed to readLine function
asis: Logical, should css pattern be matched as is (default) or permuted constructTable:
Logical, should a data frame be created, defaults to FALSE withOutTags: Logical, should
HTML tags be removed, defaults to FALSE
webScraper <- function(url, css, ..., constructTable = FALSE, withOutTags =
FALSE) {
if (is.null(get0("dom", envir = globalenv()))) {
cat("Opening", url, "n")
assign("dom", readLines(url, ...), globalenv())
} else {
if (identical(get0("dom", globalenv()), readLines(url, ...))) {
dom <- get0("dom", globalenv())
} else {
cat("Opening", url, "n")
assign("dom", readLines(url, ...), globalenv())
}
}
m <-
grepl("(^(*s)?w*$)|(^*$)|(^(*s)?w*([[^]]+])+$)*|(^(*s)?
w*([.](w*[[:punct:]]*)*)+$)+|(^(*s)?w*#(w*[[:punct:]]*)+$)+|(^(
*s)?w*:(w*[[:punct:]]*)+$)*", x = css)
if (m) {
simpleSelectors(css, doc, constructTable, withOutTags)
}
}
2. Check HTML element
Description
Check if an object is a valid HTML element.

2. Usage
is.htmlElement(x, doc = NULL)
Arguments
x A character string with HTML tags, or matrix with indices and position to extract content.
doc R object with valid HTML elements used to extract content when "x" is a matrix
otherwise NULL if "x" is a character vector.
is.htmlElement <- function(x, doc = NULL) {
if (class(x) == "matrix") {
if (is.null(doc)) stop("Please provide document to extract from")
x <- contentExtractor(x, doc)
}
if (length(x) > 1) x <- paste0(x, collapse = "n")
if (grepl("<.+/>$", x)) {
cat("Self-closing elementnn")
return(TRUE)
}
openings <- length(gregexpr("<(?!/)[^>]+(?<!/)>", x, perl = TRUE))
closings <- length(gregexpr("</[^>]+>", x))
equalTags <- openings == closings
sameName <- grepl("^<(w+b)[^>]*>.*</1>$", x)
if (equalTags && sameName) {
return(TRUE)
} else {
return(FALSE)
}
}
3. Tag Counter
Description
Count number of tags in a string
Usage
tagCounter(tag, string, start = 1, count = FALSE)
Arguments
tag a character vector with tag name. Add "/" before a tag name if counting a closing tag.
string a character string used for the search
start Integer giving exact location where "<" for the tag begins count Logical, if TRUE
returns an integer value for number of matches. If FALSE (default), return a matrix with all
of matches, there positions and length.
tagCounter <- function(tag, string, start = 1, count = FALSE) {
if (start != 1) {

3. string <- substr(string, start, nchar(string))
}
pattern = paste0("<", tag, "b[^>]*>")
matches = gregexpr(pattern = pattern, text = string)[[1]]
if (start != 1) {
position <- as.vector(matches) + (start - 1)
} else {
position <- as.vector(matches)
}
if (length(position) == 1 && position < 0) {
return(0)
}
length <- attr(matches, "match.length")
tagMat <- matrix(c(position, length), ncol = 2, dimnames =
list(1:length(position), c("Position", "Length")))
if (count) {
return(nrow(tagMat))
} else {
return(tagMat)
}
}
4. Closing tag Locator Functions
Description
"clsTagLocator" locates closing tags given position and name of an opening tag.
"multiClsTagLocator" locates closing tags for multiple opening tags.
Usage
clsTagLocator(tagName, doc, index = 1, startPos = 1) multiClsTagLocator(tagNames, doc,
indices = 1, startPos = 1)
Arguments
tagName(s) A character string for clsTagLocator and a character vector for
multiClsTagLocator. doc A valid HTML document object index/indices integer for
clsTagLocator or an integer vector of length greater than one for multiClsTagLocator. These
give index/indices of opening tags when "doc" is a multi string object
startPos an integer vector of length one or more giving start position for opening tag(s)
clsTagLocator <- function(tagName, doc, index = 1, startPos = 1) {
lengthtagName <- nchar(tagName)
if (length(doc) == 1) {
multi <- FALSE
if (index != 1) warning("index > 1 when length(doc) = 1 is not useful")
tag <- substr(doc, startPos, startPos + lengthtagName)
if (tag != paste0("<", tagName)) stop('There is no "<', tagName, '"
starting at position ', startPos, '. Start position must be at angle "<"

4. bracket and not the tag name.')
} else if (length(doc) > 1) {
if (grepl(paste0("<", tagName), doc[index])) {
locations <- as.vector(gregexpr(paste0("<", tagName),
doc[index])[[1]])
if (!any(locations == startPos)) {
warning('There is no "', tagName, '" at position ', startPos, ".
'startPos' has been set to ", locations[1])
startPos <- locations[1]
}
} else stop('Closing tag error: There is no match for <"', tagName, '"
at index ', index)
openingPos <- startPos
multi <- TRUE
multiDoc <- doc
doc <- paste0(doc, collapse = "n")
startPos <- (as.vector(gregexpr("n", doc)[[1]])[index - 1] + 1) +
(startPos - 1)
}
nCharDoc <- nchar(doc)
docSub <- substr(doc, startPos, nCharDoc)
pattern1 <- paste0("<", tagName, "b[^>]*/>")
pattern2 <- paste0("<", tagName, "b[^>]*>[^<]*</", tagName, ">")
pattern3 <- paste0("<", tagName, "b[^>]*>([^<]*<(?!/", tagName,
")[^>]+>)*?<", tagName, "[^>]*>")
if (as.vector(regexpr(pattern1, docSub)) == 1) {
cat("A self-closing elementnn")
if (multi) {
data <- c(index, index, openingPos, 0)
clsTagMat <- matrix(data, ncol = 4, byrow = TRUE, dimnames =
list("Single", c("OpeningIndex", "ClosingIndex", "OpeningPos",
"ClosingPos")))
} else {
data <- c(index, index, startPos, 0)
clsTagMat <- matrix(data, ncol = 4, byrow = TRUE, dimnames =
list("Single", c("OpeningIndex", "ClosingIndex","StartPos", "ClosingPos")))
}
return(clsTagMat)
} else if (as.vector(regexpr(pattern2, docSub)) == 1) {
m <- regexpr(pattern2, docSub)
} else if (as.vector(regexpr(pattern3, docSub, perl = TRUE)) == 1) {
pattern <- paste0("<", tagName, "b[^>]*>([^<]*<[^>]+>)*?</",
tagName, ">")
m <- regexpr(pattern, docSub)
} else {
pattern <- paste0("<", tagName, "b[^>]*>([^<]*(<[^>]+>)*)*?</",
tagName, ">")
m <- regexpr(pattern, docSub)
}
elementLength <- attr(m, "match.length")

5. closingPos <- startPos + (elementLength - 1)
if (multi) {
closingIndex <- length(gregexpr("n", substr(doc, 1, closingPos))[[1]])
+ 1
if (index == closingIndex) {
closingPos <- elementLength + (openingPos - 1)
clsTagMat <- matrix(c(index, index, openingPos, closingPos), ncol =
4, byrow = TRUE, dimnames = list("Single", c("OpeningIndex", "ClosingIndex",
"OpeningPos", "ClosingPos")))
} else {
clsChars <- closingPos - (lengthtagName + 2)
pos <- gregexpr(paste0("</", tagName, ">"), substr(doc, clsChars,
closingPos))[[1]]
if (grepl("^n", substr(doc, clsChars, closingPos))) {
closingPos <- (as.vector(pos) - 1) + (attr(pos, "match.length") - 1)
} else {
closingPos <- as.vector(pos) + (attr(pos, "match.length") - 1)
}
clsTagMat <- matrix(c(index, closingIndex, openingPos, closingPos),
ncol = 4, dimnames = list("Multi", c("OpeningIndex", "ClosingIndex",
"OpeningPos", "ClosingPos")))
}
} else {
clsTagMat <- matrix(c(index, index, startPos, closingPos), ncol = 4,
byrow = TRUE, dimnames = list("Single", c("OpeningIndex", "ClosingIndex",
"OpeningPos", "ClosingPos")))
}
clsTagMat
}
multiClsTagLocator <- function(tagNames, doc, indices = 1, startPos = 1) {
nTagNames <- length(tagNames)
nIndices <- length(indices)
nStartPos <- length(startPos)
if (length(doc) == 1) {
if (nTagNames != nStartPos && nStartPos > 1) {
tagNames <- rep(tagNames, length.out = nStartPos)
}
clsTagList <- lapply(1:nStartPos, function(i)
clsTagLocator(tagNames[i], doc, startPos = startPos[i]))
} else {
if (nIndices == 1 && nStartPos > 1) {
if (nTagNames != nStartPos) tagNames <- rep(tagNames, length.out =
nStartPos)
indices <- rep(indices, length.out = nStartPos)
} else {
if (nTagNames != nIndices && nIndices > 1) {
tagNames <- rep(tagNames, length.out = nIndices)
}
if (nStartPos != nIndices && nIndices > 1) {

6. startPos <- rep(startPos, length.out = nIndices)
}
}
nIndices <- length(indices)
clsTagList <- lapply(1:nIndices, function(i) clsTagLocator(tagNames[i],
doc, indices[i], startPos[i]))
}
Reduce("rbind", clsTagList)
}
5. Content extractor
Description
Given indices and location of opening and closing tags, it extracts HTML elements and can
either produce a data frame or remove HTML tags.
Usage
contentExtractor(x, doc, constructTable = FALSE, withOutTags = FALSE, encoding = "UTF-
8")
Arguments
x a matrix with indices and position of opeining and closing tags doc a valid HTML
document object constructTable logical; whether a data frame should be created. Defaults
to FALSE. withOutTags logical; should HTML tags be removed. Defaults to FALSE.
contentExtractor <- function(x, doc, constructTable = FALSE, withOutTags =
FALSE, encoding = "UTF-8") {
if (class(x) != "matrix") stop('"x" must be a matrix')
if (is.null(rownames(x))) stop("'rownames(x)' must be either 'Multi' or
'Single'")
rows <- nrow(x)
content <- lapply(1:rows, function(i) {
if (rownames(x)[i] == "Multi") {
multi <- doc[x[i, "OpeningIndex"]:x[i, "ClosingIndex"]]
multi[[1]] <- substr(multi[[1]], x[i, "OpeningPos"],
nchar(multi[[1]]))
multi[[length(multi)]] <- substr(multi[[length(multi)]], 1, x[i,
"ClosingPos"])
multi
} else {
substr(doc[x[i, "OpeningIndex"]], x[i, "OpeningPos"], x[i,
"ClosingPos"])
}
})
for (i in 1:rows) {
Encoding(content[[i]]) <- encoding
}
if (length(content) == 1) {

7. content <- content[[1]]
}
if (constructTable) {
if (rows == 1) {
return(tableConstructor(content))
} else {
return(multiTableConstructor(content))
}
} else if (withOutTags) {
content <- as.vector(sapply(content, gsub, pattern = '</?[^>]*>',
replacement = ""))
logi <- sapply(1:length(content), function(i) grepl(pattern = "w+", x
= content[[i]]))
return(lapply(1:length(logi), function(i)
content[[i]][which(logi[[i]])]))
} else {
return(content)
}
}
6. Table Constructor
Description
Creates a data frame from a html table element.
Usage
tableConstructor(x, encoding = "UTF-8")
Arguments
x A table element encoding Encoding to be set for all variables
tableConstructor <- function(x, encoding = "UTF-8") {
indices <- grep("<tr", x)
nIndices <- length(indices)
trOpCls <- multiClsTagLocator("tr", doc = x, indices = indices)
rawTr <- contentExtractor(trOpCls, x)
nRows <- if (any(grepl("th", rawTr[[1]]))) {
nIndices - 1
} else {
nIndices
}
nCols <- sapply(seq(rawTr), function(i) length(grep("<t(h|d)",
rawTr[[i]])))
uniqCol = unique(nCols)
nCols = nCols[which.max(nCols)]
if (length(uniqCol) > 1) {
warning("There are cell data spanning more than one column")
}

8. df = data.frame(matrix(nrow = nRows, ncol = nCols))
if (any(grepl("th", rawTr[[1]]))) {
th = grep("<[^>]*th", rawTr[[1]])
colNams = gsub("<s*/?[^>]*>", "", rawTr[[1]][th])
if (length(colNams) != nCols) {
names(df) = paste0("Var", seq(nCols))
} else {
names(df) = colNams
rawTr = rawTr[-1]
}
} else {
names(df) = paste0("Var", seq(nCols))
}
for (i in 1:length(rawTr)) {
ind <- grep("<t(h|d)", rawTr[[i]])
for (j in 1:length(ind)) {
tag <- sub("<(t(h|d))[^>]*>.*", "1", rawTr[[i]][ind[j]])
element <- clsTagLocator(tag, rawTr[[i]], ind[j])
ij = paste(contentExtractor(element, rawTr[[i]], withOutTags =
TRUE), collapse = "; ")
Encoding(ij) = encoding
df[i, j] = gsub("&#160;", " ", ij)
}
}
df
}
multiTableConstructor <- function(x, encoding = "UTF-8") {
tables <- vector("list", length(x))
for (i in 1:length(x)) {
tables[[i]] <- tableConstructor(x[[i]], encoding = "UTF-8")
}
tables
}
7. Content Remover
Description
Removes unwanted content from outputs of content extractor or from columns in created
data frames.
Usage
contentRemover(x, content, column = NULL)

9. Arguments
x a charcter string, a data frame or a list with data frames. content charcter string with any
regular expression including literals and special characters targeting content to be
removed column integer vector indicating one or more columns for which specified content
will be removed.
Value
If "x" is a data frame, then a data frame is returned, if it is a list, a list with data frames will
be outputed.
contentRemover <- function(x, content, column = NULL) {
removeContent <- function(x, content, column = NULL) {
if (length(column) == 1) {
x[, column] <- gsub(content, "", x[, column])
} else {
for (i in seq(column)) {
x[, column[i]] <- gsub(content, "", x[, column[i]])
}
}
x
}
if (class(x) == "data.frame") {
return(removeContent(x, column, content))
} else if (class(x) == "list") {
elements <- vector("list", length(x))
for (i in seq(x)) {
if (class(x[[i]]) == "data.frame") {
elements[[i]] <- removeContent(x = x[[i]], column, content)
} else {
elements[[i]] <- gsub(pattern = content, replacement = "", x =
x[[i]])
}
}
if (class(elements[[1]]) != "data.frame") {
elements <- sapply(seq(elements), function(i)
elements[[i]][which(nchar(elements[[i]]) > 0)])
}
return(elements)
} else {
return(gsub(pattern = content, replacement = "", x = x))
}
}

10. 8. Attribute pattern constructor
Description
Constructs a search pattern for attributes based on given css.
Usage
attrPatternConstructor(css, asis = TRUE)
Arguments
css character string with cascading styling sheet selector for which an attibute pattern will
be constructed. asis logical; if TRUE (default), it will construct a pattern using the given
order of attributes. If FALSE, an alternating pattern will be constructed out of all
permutations of listed attributes.
attrPatternConstructor <- function(css, asis = TRUE) {
if (!grepl("[.[#]", css)) {
if (grepl("w+", css)) cat("Detecting tag name onlyn")
stop("No attributes listed")
}
cssAttributes <- vector("list")
counter <- 0
pattern1 <- "(?<=[.])[^.[#]+"
pattern2 <- "(?<=#)[^.[#]+"
pattern3 <- "(?<=[)(.+?)(?=])"
if (grepl(pattern1, css, perl = TRUE)) {
counter <- counter + 1
classes <- regmatches(css, gregexpr(pattern1, css, perl = TRUE))[[1]]
classes <- sub(pattern1, "1", classes, perl = TRUE)
withClass <- TRUE
cssAttributes[[counter]] <- paste0('class="', paste(classes, collapse =
" "), '"')
} else withClass <- FALSE
if (grepl(pattern2, css, perl = TRUE)) {
counter <- counter + 1
if (length(gregexpr(pattern2, css, perl = TRUE)[[1]]) > 1)
warning("Elements can only have one 'id' attribute, hence only the first is
matched")
id <- regmatches(css, regexpr(pattern2, css, perl = TRUE))
withId <- TRUE
cssAttributes[[counter]] <- paste0('id="', sub(pattern2, "1", id,
perl = TRUE), '"')
} else withId <- FALSE
if (grepl(pattern3, css, perl = TRUE)) {
pattn1 <- '[class(="([[:graph:]]+)")?]'
pattn2 <- '[id(="[[:graph:]]+")?]'
if (withClass && grepl(pattn1, css)) {
additionalClasses <- regmatches(css, regexpr(pattn1, css))[[1]]

11. pattn <- '[class="([^"]*)"]'
if (grepl(pattn, additionalClasses)) {
additionalClasses <- sub('[class="([^"]*)"]', "1",
additionalClasses)
}
cssAttributes[[1]] <- paste0('class="', paste(classes,
additionalClasses, collapse = " "), '"')
css <- regmatches(css, regexpr(pattn1, css), invert = TRUE)[[1]]
css <- paste(css, collapse = "")
}
if (withId && grepl(pattn2, css)) {
warning("More than one version of element 'id' given, only the first
with '#' is used")
css <- regmatches(css, regexpr(pattn2, css), invert = TRUE)[[1]]
css <- paste(css, collapse = "")
}
}
if (grepl(pattern3, css, perl = TRUE)) {
attrb <- regmatches(css, gregexpr(pattern3, css, perl = TRUE))[[1]]
if (length(grep("=", attrb, invert = TRUE))) {
counter <- counter + 1
ind <- grep("=", attrb, invert = TRUE)
cssAttributes[[counter]] <- paste0(attrb[ind], '="[^"]+"')
}
pattern4 <- '([^=~|^$*]+)([~|^$*]?)="(.+)"'
if (any(grepl(pattern4, attrb))) {
ind <- grep(pattern4, attrb)
componentTwo <- sub(pattern4, "1", attrb[ind])
extra <- sub(pattern4, "2", attrb[ind])
nExtra <- length(extra)
value <- sub(pattern4, "3", attrb[ind])
val <- rep(NA, length(extra))
if (any(extra == "")) {
ind <- which(extra == "")
val[ind] <- paste0('"', value[ind], '"')
}
if (any(extra == "~")) {
ind <- which(extra == "~")
val[ind] <- paste0('"([[:graph:]]*s)*?', value[ind],
'(s[[:graph:]]*)*"')
}
if (any(extra == "|")) {
ind <- which(extra == "|")
val[ind] <- paste0('"', value[ind], '(-[[:graph:]]+)?"')
}
if (any(extra == "^")) {
ind <- which(extra == "^")
val[ind] <- paste0('"', value[ind], '[[:graph:]]+"')
}
if (any(extra == "$")) {

12. ind <- which(extra == "$")
val[ind] <- paste0('"[[:graph:]]+', value[ind], '"')
}
if (any(extra == "*")) {
ind <- which(extra == "*")
val[ind] <- paste0('"[[:graph:]]*', value[ind], '([[:graph:]]*"')
}
counter <- counter + 1
cssAttributes[[counter]] <- paste(componentTwo, val, sep = "=")
}
}
cssAttributes <- unlist(cssAttributes)
if (is.null(cssAttributes)) {
return(cssAttributes)
}
n <- length(cssAttributes)
f <- factorial(n)
cl <- "s([^s]+s)*?"
if (n == 1 | asis) {
pattern <- cssAttributes
if (length(pattern) > 1) {
pattern <- paste(pattern, collapse = " ")
}
} else {
indMatrix <- permutationTuples(n)
patternList <- lapply(1:f, function(i)
paste(cssAttributes[indMatrix[i,]], collapse = cl))
pattern <- unlist(patternList)
pattern <-paste(pattern, collapse = "|")
}
pattern
}
9. Permutation Tuples
Description
Generates a matrix with all permutation tuples given an integer.
Usage
permutationTuples(n)
Arguments
n integer vector of length one from which permutation tuples will be generated.
permutationTuples <- function(n) {
if (!is.numeric(n) | length(n) > 1) stop('"n" must be a numeric vector of
length one')
if (grepl("[.]", n)) {

13. warning('"n" is a float point number, it has be rounded up to ',
ceiling(n))
n <- ceiling(n)
}
permMat <- matrix(0,nrow = factorial(n), ncol = n)
i <- 0
repeat {
perm <- sample(n)
logi <- sapply(1:factorial(n), function(i) !all(permMat[i,] == perm))
if (all(logi)) {
i <- i + 1
permMat[i,] <- perm
}
if (i == factorial(n)) {break}
}
permMat
}
10. Simple Selector
Description
Based on simple selectors (part of css selectors), it produced targeted content as is or
without HTML tags or if a table element, can create a data frame.
Usage
simpleSelectors(css, doc, asis = TRUE, content = TRUE, constructTable = FALSE,
withOutTags = FALSE, encoding = "UTF-8")
Arguments
css character string with simple selector which include, type, universal, class, id and
attributes. Pseudo classes are currently (pre-aplha version: 0.0.0) not supported (but
underdevelopment). doc a HTML valid document object from which data will be extracted
asis logical; should css be used as is (default) or attributes permuted content logical; should
content of matched selector be returned (default) or tag names and indices of their match
constructTable logical; should a table be constructed if it is a valid HTML table element,
defaults to FALSE withOutTags logical; should HTML tags be removed, defaults to FALSE
encoding character string giving encoding to be applied to content.
simpleSelectors <- function(css, doc, asis = TRUE, content = TRUE,
constructTable = FALSE, withOutTags = FALSE, encoding = "UTF-8") {
if (grepl("^*$", css)) return(doc)
pattern <- "([^:]+):(w+$|w+-w+(-w+-?w*(([^)]+))?)?)"
if (grepl(pattern, css)) {
withPseudo <- TRUE
css <- sub(pattern, "1", css)
pseudoClass <- sub(pattern, "2", css)
} else withPseudo <- FALSE

14. if (grepl("^(*s)?w+$", css)) {
tagName <- sub("^(*s)?(w+)$", "2", css)
if (all(grepl(paste0("<", tagName), doc) == FALSE)) stop("No match
found for <", tagName)
indices <- grep(paste0("<", tagName), doc)
if (!content) return(list(tagNames = tagName, indices = indices))
stp <- sapply(indices, function(i) regexpr(paste0("<", tagName), text =
doc[i]))
clsTagMat <- multiClsTagLocator(tagNames = tagName, doc = doc, indices
= indices, startPos = stp)
} else {
if (grepl("^w", css)) {
pattn <- "^(w+)([.[#].*)"
tagNames <- sub(pattn, "1", css)
pattern <- attrPatternConstructor(sub(pattn, "2", css), asis)
pattern <- paste0("<", tagNames, "b[^>]*?", pattern, "[^>]*>")
if (all(grepl(pattern, doc) == FALSE)) stop("No match for ",
pattern)
indices <- grep(pattern, doc)
stp <- sapply(indices, function(i) regexpr(pattern, doc[i]))
} else {
pattern <- attrPatternConstructor(css, asis)
if (all(grepl(pattern, doc) == FALSE)) stop("No match for ",
pattern)
pattn <- paste0('<(w+b)[^>]*?', pattern, "[^>]*>.+$")
indices <-grep(pattn, doc)
tagNames <- sub(pattn, "1", doc[indices])
if (!all(sapply(tagNames, grepl, pattern = "bw+b"))) {
stop("Pattern does not match tag names, instead matches ",
tagNames)
}
stp <- sapply(indices, function(i) regexpr(pattn, doc[i]))
}
if (!content) return(list(tagNames = tagNames, indices = indices))
clsTagMat <- multiClsTagLocator(tagName = tagNames, doc = doc, indices
= indices, startPos = stp)
}
contentExtractor(x = clsTagMat, doc = doc, constructTable =
constructTable, withOutTags = withOutTags, encoding = encoding)
}
11. nth interpreter
Description
Used to compute "an+b" algebra in pseudo-class selector.
Usage
nthInterpreter(nth, nDoc, fromLast = FALSE)

15. Arguments
nth character vector with details of "an+b". Essentially what is in brackets when pseudo-
class selector begins with "nth". nDoc integer; total number of from which nth will be
compted fromLast logical; should selection be done in reverse as is the case with pseudo-
class selectors with "from-last". Default is FALSE.
nthInterpreter <- function(nth, nDoc, fromLast = FALSE) {
if (grepl("^d+$", nth)) {
return(as.integer(nth))
}
pattern <- "([+-]?)(d*)([+-]?)(n?)([+-]?)(d*)"
if (!grepl(pattern, nth) | nth == "") stop("nth is not interpretable")
if (grepl("^[+-]?n[+-]?d+", nth)) nth <- paste0("+1", nth)
a <- sub(pattern, "2", nth)
n <- sub(pattern, "4", nth)
b <- sub(pattern, "6", nth)
if (a != "" && n == "" && b != "") stop("nth not interpretable")
if (a != "" && n != "" && b == "") b <- 0
if (a == "" && n != "" && b == "") stop('"a" and "b" missing')
if (a == b && n != "") b <- 0
if (nth == "even") {
a <- "2"
n <- "n"
b <- "0"
}
if (nth == "odd") {
a <- "2"
n <- "n"
b <- "1"
}
if (!(a != "" && n != "" && b != "")) stop("nth not interpretable")
aSign <- sub(pattern, "1", nth)
nSign <- sub(pattern, "3", nth)
bSign <- sub(pattern, "5", nth)
if (all(c(aSign != "+", aSign != "-"))) aSign <- "+"
if (all(c(nSign != "+", nSign != "-"))) nSign <- "+"
if (all(c(bSign != "+", bSign != "-"))) bSign <- "+"
a <- as.numeric(paste0(aSign, a)); b <- as.numeric(paste0(bSign, b))
if (fromLast) {
n <- ceiling(as.numeric(paste0(nSign, (nDoc/a - b):0)))
} else {
n <- ceiling(as.numeric(paste0(nSign, 0:(nDoc/a - b))))
}
nthIndices <- a * n + b
nthIndices <- nthIndices[which(nthIndices > 0)]
if (length(nthIndices) == 1 && nDoc != 1 && fromLast) {
nthIndices <- (nDoc:1)[nthIndices]
}

16. nthIndices
}