fastmod

Fast, interactive codebase-wide find and replace

TLDR

Replace a string interactively across the current directory

$ fastmod "[old_string]" "[new_string]"

Replace using a regex with multiline matching

$ fastmod -m "[pattern]" "[replacement]"

Replace in a specific directory

$ fastmod "[old]" "[new]" [path/to/directory]

Replace only in files with specific extensions

$ fastmod -e "[py,js]" "[old]" "[new]"

Accept all replacements without prompting

$ fastmod --accept-all "[old]" "[new]"

Preview matches without modifying files

$ fastmod --print-only "[old]" "[new]"

Treat pattern as a literal string instead of regex

$ fastmod --fixed-strings "[old.string()]" "[new.string()]"

SYNOPSIS

fastmod [options] pattern [replacement] [paths...]

fastmod performs fast, interactive codemod operations across files. It searches for regex patterns and prompts for confirmation before each replacement, making bulk code changes safer.The tool uses the same regex engine as ripgrep for fast searching combined with interactive review of changes. By default, patterns are treated as regular expressions. If no replacement is given, fastmod deletes the matched text. If no paths are given, it searches the current directory recursively.During interactive mode, press y to accept, n to skip, e to edit the replacement, a to accept all remaining, or q to quit.

PARAMETERS

-m, --multiline

Enable multiline regex matching (dot matches newlines).

-i, --ignore-case

Case insensitive matching.

-e, --extensions exts

Comma-separated list of file extensions to process (e.g., py,js,ts).

--accept-all

Apply all replacements without interactive prompting.

--print-only

Show matches without modifying files.

-d, --dir path

Set the root search directory (default: current directory).

--glob pattern

Include/exclude files by glob pattern (prefix with ! to exclude).

--hidden

Include hidden files and directories in the search.

--fixed-strings, -F

Treat the pattern as a literal string, not a regex.

--count

Display the total number of matches/replacements.

CAVEATS

Uses Rust's regex syntax, which may differ from PCRE or POSIX regex in some edge cases. Interactive mode requires a terminal. When using --accept-all, changes are applied without confirmation -- use --print-only first to preview.

HISTORY

fastmod was developed at Facebook (Meta) as a tool for large-scale code modifications. It addresses the need for interactive, safe refactoring across massive codebases. Released as open source, it is written in Rust for performance.