grep and ripgrep

The terminal's regex tool

For shell-level text searching, the canonical tool was grep. The modern replacement is ripgrep (rg) — written in Rust, uses RE2-class engine, recursive by default, respects .gitignore, blazing fast.

grep flavors

POSIX grep has three modes that differ in how they treat metacharacters:

BRE (default): ?, +, {}, (), | are LITERAL unless escaped. grep 'foo$bar$'
ERE (grep -E or egrep): Modern syntax — those metacharacters work without escaping. grep -E 'foo(bar)'
PCRE (grep -P): Full Perl-compatible. Lookaround, backreferences, named groups. Not always available (depends on grep build).

Use -E by default. Use -P when you need PCRE features. Avoid plain grep with regex — too many gotchas.

ripgrep — the modern default

rg uses RE2-style syntax (no lookaround, no backreferences, but linear time). It's recursive by default, respects .gitignore, supports Unicode well, and is dramatically faster than grep on large codebases.

Common flags:

rg pattern — search recursively in current directory
rg -i pattern — case-insensitive
rg -n pattern — show line numbers (default in interactive mode)
rg -l pattern — list only filenames with matches
rg -A 3 pattern — show 3 lines of context after each match
rg --pcre2 pattern — enable PCRE2 (lookaround, backreferences)
rg -r 'replacement' pattern — preview replacements ($1, $2 for groups)

Code

grep vs ripgrep·bash

# POSIX grep BRE — escaping required
grep 'foo\(bar\)' file.txt

# grep ERE — modern
grep -E 'foo(bar)' file.txt

# grep PCRE — full features (when available)
grep -P '(?<=user_)\w+' file.txt

# ripgrep — recursive by default, respects .gitignore
rg 'TODO|FIXME' .

# ripgrep with line numbers and case insensitive
rg -ni 'error' logs/

# ripgrep with PCRE for lookaround
rg --pcre2 '(?<=password = )\S+' config/

# Preview a replacement
rg -r 'NEW: $1' '(\d+)' file.txt

# Find files containing a pattern
rg -l 're.compile' .

# Search only specific file types
rg -t py 'import re' .

The terminal's regex tool

grep flavors

ripgrep — the modern default

Code

External links

Exercise

Progress

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