Regex › Bash, grep, sed, awk Regex

Command-line regex: BRE, ERE, and PCRE

3 min read Intermediate 3 sections

On the command line, the same pattern can work in one tool and fail in another, because grep, sed, and awk use different regex dialects. Knowing the three — BRE, ERE, and PCRE — is what saves you from the maddening ‘my regex works in Python but not in grep’ problem.

You'll learn to

Tell BRE, ERE, and PCRE apart
Choose the right flag for each tool
Write portable command-line patterns

The three dialects

BRE (Basic):     grep, sed by default
  + ? | ( ) { } are LITERAL unless backslash-escaped: \( \) \{ \}
ERE (Extended):  grep -E, sed -E, awk
  + ? | ( ) { } work as metacharacters directly
PCRE (Perl):     grep -P (where available)
  full features: \d \w lookarounds, like Python/JS

The big gotcha is BRE: in basic mode, +, ?, |, (), and {} are literal characters, and you must backslash-escape them to get their special meaning. In ERE (-E), they work directly. PCRE (-P) adds the full Perl features like \d and lookarounds.

The same pattern, three ways

# Match one or more digits:
grep    '[0-9][0-9]*'   file     # BRE: no + available, so repeat manually
grep -E '[0-9]+'        file     # ERE: + works
grep -P '\d+'           file     # PCRE: \d works

# Alternation:
grep -E 'cat|dog' file           # ERE
grep    'cat\|dog' file          # BRE: escape the pipe

Checkpoint

Why does grep '[0-9]+' (without -E) often return nothing, while grep -E '[0-9]+' works?

Try it yourself

Write a pattern to match one or more digits three ways: in BRE (plain grep), in ERE (grep -E), and in PCRE (grep -P). Then write an alternation matching ‘cat’ or ‘dog’ in both BRE and ERE, noting where you must escape the pipe.

Key takeaways

BRE (plain grep/sed): + ? | ( ) are literal unless escaped.
ERE (grep -E, awk): those metacharacters work directly.
PCRE (grep -P): full Perl features like \d and lookarounds.
Default to -E; suspect the dialect when a pattern matches nothing.

Quick quiz

Next, Go regex — the RE2 engine and its linear-time guarantee for security tooling.

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