Title:	Parse Front Matter from Documents
Version:	0.2.0
Description:	Extracts and parses structured metadata ('YAML' or 'TOML') from the beginning of text documents. Front matter is a common pattern in 'Quarto' documents, 'R Markdown' documents, static site generators, documentation systems, content management tools and even 'Python' and 'R' scripts where metadata is placed at the top of a document, separated from the main content by delimiter fences.
License:	MIT + file LICENSE
URL:	https://github.com/posit-dev/frontmatter, https://posit-dev.github.io/frontmatter/
BugReports:	https://github.com/posit-dev/frontmatter/issues
Imports:	cpp11, rlang, tomledit, yaml12
Suggests:	testthat (≥ 3.0.0), withr, yaml
LinkingTo:	cpp11
Config/Needs/website:	brand.yml
Config/testthat/edition:	3
Encoding:	UTF-8
RoxygenNote:	7.3.3
NeedsCompilation:	yes
Packaged:	2026-02-09 19:32:24 UTC; garrick
Author:	Garrick Aden-Buie [aut, cre], Posit Software, PBC [cph, fnd]
Maintainer:	Garrick Aden-Buie <garrick@posit.co>
Repository:	CRAN
Date/Publication:	2026-02-09 19:50:02 UTC

frontmatter: Parse Front Matter from Documents

Description

Extracts and parses YAML or TOML front matter from text documents. Front matter is structured metadata at the beginning of a document, delimited by fences.

Supported Formats

• Standard YAML (⁠---⁠ delimiters)

• Standard TOML (⁠+++⁠ delimiters)

• Comment-wrapped formats for R/Python files (⁠#⁠ and ⁠#'⁠ prefixes)

• PEP 723 Python inline script metadata

Main Functions

• parse_front_matter(): Parse front matter from a string

• read_front_matter(): Parse front matter from a file

Performance

Uses C++11 for fast, single-pass parsing with minimal memory overhead. Designed for high throughput processing of many documents.

Author(s)

Maintainer: Garrick Aden-Buie garrick@posit.co (ORCID)

Other contributors:

• Posit Software, PBC (ROR) [copyright holder, funder]

See Also

Useful links:

• https://github.com/posit-dev/frontmatter

• https://posit-dev.github.io/frontmatter/

• Report bugs at https://github.com/posit-dev/frontmatter/issues

Format and Write YAML or TOML Front Matter

Description

Serialize R data as YAML or TOML front matter and combine it with document content. format_front_matter() returns the formatted document as a string, while write_front_matter() writes it to a file or prints to the console. These functions are the inverse of parse_front_matter() and read_front_matter().

Usage

format_front_matter(
  x,
  delimiter = "yaml",
  format = "auto",
  format_yaml = NULL,
  format_toml = NULL
)

write_front_matter(
  x,
  path = NULL,
  delimiter = "yaml",
  ...,
  format = "auto",
  format_yaml = NULL,
  format_toml = NULL
)

Arguments

Value

• format_front_matter(): A character string containing the formatted document with front matter.

• write_front_matter(): Called for its side effect; returns NULL invisibly.

Functions

• format_front_matter(): Format front matter as a string

• write_front_matter(): Write front matter to a file or console

Delimiter Formats

The delimiter argument controls the fence style used to wrap the front matter. You can use these built-in shortcuts:

For custom delimiters, pass a character vector of length 1, 2, or 3:

• Length 1: Used as both opener and closer, with no line prefix

• Length 2: c(opener, prefix) where opener is also used as closer

• Length 3: c(opener, prefix, closer) for full control

Custom Formatters

By default, the package uses yaml12::format_yaml() for YAML and tomledit::to_toml() for TOML. You can provide custom formatter functions via format_yaml and format_toml to override these defaults.

Custom formatters must accept an R object and return a character string containing the serialized content.

YAML Specification Version

The default YAML formatter uses YAML 1.2 via yaml12::format_yaml(). To use YAML 1.1 formatting instead (via yaml::as.yaml()), set either:

• The R option frontmatter.serialize_yaml.spec to "1.1"

• The environment variable FRONTMATTER_SERIALIZE_YAML_SPEC to "1.1"

The option takes precedence over the environment variable. Valid values are "1.1" and "1.2" (the default).

Roundtrip Support

Documents formatted with these functions can be read back with parse_front_matter() or read_front_matter(). For comment-prefixed formats (like yaml_comment or yaml_roxy), a separator line is automatically inserted between the closing fence and the body when the body starts with the same comment prefix, ensuring clean roundtrip behavior.

See Also

parse_front_matter() and read_front_matter() for the inverse operations.

Examples

# Create a document with YAML front matter
doc <- list(
  data = list(title = "My Document", author = "Jane Doe"),
  body = "Document content goes here."
)

# Format as a string
format_front_matter(doc)

# Write to a file
tmp <- tempfile(fileext = ".md")
write_front_matter(doc, tmp)
readLines(tmp)

# Print to console (when path is NULL)
write_front_matter(doc)

# Use TOML format
format_front_matter(doc, delimiter = "toml")

# Use comment-wrapped format for R scripts
r_script <- list(
  data = list(title = "Analysis Script"),
  body = "# Load libraries\nlibrary(dplyr)"
)
format_front_matter(r_script, delimiter = "yaml_comment")

# Roundtrip example: read, modify, write
original <- "---
title: Original
---
Content here"

doc <- parse_front_matter(original)
doc$data$title <- "Modified"
format_front_matter(doc)

Parse YAML or TOML Front Matter

Description

Extract and parse YAML or TOML front matter from a file or a text string. Front matter is structured metadata at the beginning of a document, delimited by fences (⁠---⁠ for YAML, ⁠+++⁠ for TOML). parse_front_matter() processes a character string, while read_front_matter() reads from a file. Both functions return a list with the parsed front matter and the document body.

Usage

parse_front_matter(text, parse_yaml = NULL, parse_toml = NULL)

read_front_matter(path, parse_yaml = NULL, parse_toml = NULL)

Arguments

Value

A named list with two elements:

• data: The parsed front matter as an R object, or NULL if no valid front matter was found.

• body: The document content after the front matter, with leading empty lines removed. If no front matter is found, this is the original text.

Functions

• parse_front_matter(): Parse front matter from text

• read_front_matter(): Parse front matter from a file.

Custom Parsers

By default, the package uses yaml12::parse_yaml() for YAML and tomledit::parse_toml() for TOML. You can provide custom parser functions via parse_yaml and parse_toml to override these defaults.

Use identity to return the raw YAML or TOML string without parsing.

YAML Specification Version

The default YAML parser uses YAML 1.2 via yaml12::parse_yaml(). To use YAML 1.1 parsing instead (via yaml::yaml.load()), set either:

• The R option frontmatter.parse_yaml.spec to "1.1"

• The environment variable FRONTMATTER_PARSE_YAML_SPEC to "1.1"

The option takes precedence over the environment variable. Valid values are "1.1" and "1.2" (the default).

YAML 1.1 differs from YAML 1.2 in several ways, most notably in how it handles boolean values (e.g., yes/no are booleans in 1.1 but strings in 1.2).

Examples

# Parse YAML front matter
text <- "---
title: My Document
date: 2024-01-01
---
Document content here"

result <- parse_front_matter(text)
result$data$title  # "My Document"
result$body        # "Document content here"

# Parse TOML front matter
text <- "+++
title = 'My Document'
date = 2024-01-01
+++
Document content"

result <- parse_front_matter(text)

# Get raw YAML without parsing
result <- parse_front_matter(text, parse_yaml = identity)

# Use a custom parser that adds metadata
result <- parse_front_matter(
  text,
  parse_yaml = function(x) {
    data <- yaml12::parse_yaml(x)
    data$parsed_at <- Sys.time()
    data
  }
)

# Or read from a file
tmpfile <- tempfile(fileext = ".md")
writeLines(text, tmpfile)

read_front_matter(tmpfile)