grep reads files (or standard input) and prints the lines that match a pattern. For ordinary text searches, use a quoted basic regular expression (BRE), or add -E for extended regular expressions (ERE) when you need readable grouping, alternation, or repetition. For example, grep -E 'error|failed' app.log prints every line containing either word.
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This guide explains the regex dialects built into grep, shell quoting, precise line and word matching, multiple patterns, recursive searches, locale and binary-data behavior, and the common mistakes that produce surprising results.
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GNU grep searches named input files—or standard input when no file is named—for lines containing a match to the supplied pattern. It normally writes each selected line to standard output, including the line’s original newline. A pattern is not required to match the entire line: unless you add anchors or a whole-line option, a match anywhere in the line selects it.
The general form is:
grep [options] 'pattern' file...Examples:
# Search one file with the default regular-expression dialect
grep 'timeout' server.log
# Read from a pipeline instead of a named file
journalctl -b | grep -i 'failed'Use single quotes around a pattern in shell commands. They prevent the shell from interpreting characters such as $, *, ?, parentheses, brackets, and the backslash before grep receives them. If you deliberately need shell variable expansion, use double quotes and understand which characters the shell will still process.
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Choose the grep matcher you actually need
GNU grep provides four matcher families. The option changes how the pattern is interpreted; it does not change the input files or the output format.
| Matcher | Option | Best for | Important behavior |
|---|---|---|---|
| Basic regular expressions (BRE) | -G, or no option |
Portable traditional grep patterns | Grouping, alternation, and interval repetition commonly need backslashes. |
| Extended regular expressions (ERE) | -E |
Readable alternatives, groups, and repetitions | (...), |, and {m,n} have their usual unescaped meanings. |
| Fixed strings | -F |
Literal text, not patterns | Metacharacters lose their regex meaning. This is often faster and safer for user-supplied text. |
| Perl-compatible regular expressions (PCRE) | -P |
Features not available in POSIX BRE or ERE | Availability and warnings vary by GNU grep build; portability is lower. |
Use the least powerful matcher that expresses the requirement. Choose -F when you need an exact literal substring, the default BRE for simple portable patterns, ERE for clear alternation and grouping, and PCRE only when a specific Perl-compatible feature justifies its portability cost.
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Suppose you need to find the literal text a.b. A regex interprets the dot as “any character,” so use fixed-string mode:
grep -F 'a.b' notes.txtAlternatively, escape the dot in a regex:
grep 'a.b' notes.txtFor a list of strings supplied by another person or program, -F avoids treating punctuation as executable pattern syntax.
Regex building blocks for grep
grep tests a pattern against each input line (excluding the terminating newline). The following constructs cover most log, configuration, and source-code searches.
| Construct | Meaning | Example |
|---|---|---|
^ |
Beginning of the line | ^ERROR finds lines that start with ERROR. |
$ |
End of the line | done$ finds lines ending in done. |
. |
Any single character | gr.y can match gray or grey. |
[abc] |
One character from the bracket expression | gr[ae]y matches gray or grey. |
[^abc] |
One character not in the bracket expression | [^0-9] matches a non-digit character. |
[0-9] |
A character in the stated range | [0-9][0-9] matches two digits. |
[[:digit:]] |
A POSIX character class | ^[[:digit:]] finds lines beginning with a digit. |
* |
Zero or more repetitions of the preceding atom | ab*c matches ac, abc, and abbc. |
|
Escapes a metacharacter or gives special meaning in BRE | . matches a literal dot. |
A bracket expression is one character, not a complete word. Thus [0-9] matches one digit; append a repetition or additional expressions when you need a multi-digit value. POSIX classes such as [[:digit:]], [[:lower:]], and [[:space:]] are preferable when locale portability matters.
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BRE is grep’s default. ERE, enabled with -E, makes several operators easier to read. The underlying idea is the same, but the characters that activate grouping, alternation, and interval repetition differ.
| Intent | BRE | ERE |
|---|---|---|
| Group two alternatives | (cat|dog) |
(cat|dog) |
| Either word | cat|dog |
cat|dog |
| Repeat two to four times | {2,4} |
{2,4} |
For example, these commands select lines beginning with either ERROR: or WARN::
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# ERE: usually the clearest form
grep -E '^(ERROR|WARN):' app.log
# BRE equivalent
grep '^(ERROR|WARN):' app.logUse ERE when the pattern contains several alternatives or nested groups. Use BRE when a small, traditional expression is sufficient or when you are targeting environments where default grep behavior is expected. Do not mix the two syntaxes casually: an ERE written with BRE escapes can match different text than intended.
Everyday search recipes
Find a simple term
grep 'timeout' server.logThis selects every line containing the character sequence timeout, regardless of where it occurs in the line. Matching is case-sensitive unless you add -i.
Ignore case
grep -i 'warning' server.log-i performs case-insensitive matching. Combine it with other options when needed:
grep -iw 'warning' server.logThis finds a word such as warning or Warning, but not a longer word such as prewarning.
Search for alternatives
grep -E 'timeout|connection refused' server.logThe vertical bar means “or” in ERE. Quote the complete expression so the shell passes the bar to grep. Parentheses control which part is grouped:
grep -E '^(ERROR|WARN):' app.logWithout the parentheses, the anchor would apply only to the first alternative in many expressions, producing a broader match than intended.
Match a whole word
grep -w 'warning' server.log-w asks grep to select matches that form whole words according to grep’s word-character rules. It is useful for identifiers and status words, but it is not a universal natural-language tokenizer: punctuation and non-ASCII word boundaries can depend on the locale.
Match an entire line
grep -x 'READY' status.txt-x selects only lines whose complete contents match the pattern. It is equivalent in intent to adding both anchors:
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grep '^READY$' status.txtThe option is easier to read when the requirement is explicitly “the entire line.”
Select lines that do not match
grep -v '^#' config.conf-v inverts selection. The command above prints lines that do not begin with #; it does not remove blank lines. To ignore both comments and empty lines, use an ERE that matches either case, then invert it:
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grep -Ev '^(#|$)' config.confCombine precision options
Options can be combined when each answers a different question. For example, this finds whole-word occurrences of either error or failed, ignoring case:
grep -Eiw 'error|failed' app.logKeep the pattern quoted even when it contains no obvious shell metacharacters; it makes later edits safer.
Multiple patterns and patterns that begin with a hyphen
Repeat -e for a short list
grep -e 'timeout' -e 'connection refused' server.logEach -e supplies one pattern, and a line matching any supplied pattern is selected. This form is convenient when the alternatives are assembled by a script or when each pattern deserves separate quoting.
Read patterns from a file
grep -f patterns.txt access.logpatterns.txt contains one pattern per line. The patterns are interpreted using the selected matcher, so add -E if the file contains ERE syntax:
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grep -E -f patterns.txt access.logBlank lines and whitespace in the pattern file are significant in ways that depend on the exact pattern; create the file deliberately and inspect it when a result seems unexpectedly broad.
Protect a pattern that starts with -
A pattern beginning with a hyphen can be mistaken for an option. Use -e:
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grep -e '-pattern' file.txtOn systems that support the conventional option terminator, this also works:
grep -- '-pattern' file.txt-e is explicit and remains useful when several patterns are present.
Searching directories and controlling the input
grep can search more than one file. Supplying multiple names prints the filename before each matching line, which helps identify the source:
grep -E 'ERROR|WARN' app.log worker.logFor directory trees, GNU grep supports recursive options. -r follows symbolic links encountered while traversing directories according to grep’s recursive behavior; -R follows all symbolic links. Use recursion intentionally, because it can include generated files, caches, binary files, and very large trees.
grep -RIn --include='*.conf' 'timeout' /etc/myapp-Rsearches recursively while following symbolic links.-Iskips binary files.-nprints the matching line number.--include='*.conf'limits traversal results to names matching the shell-style file pattern.
The --include value is a filename filter, not a regular expression. Keep its wildcard quoted so the current shell does not expand it before grep sees it. If you need to exclude a directory or filename pattern, GNU grep also provides corresponding --exclude and --exclude-dir options.
Useful output and status controls
grep’s output options let you turn a search into a script-friendly test or a compact report.
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fiLocale, character classes, and binary data
Locale affects ranges and classes
Character ranges and POSIX classes are interpreted in the current locale. A range such as [a-z] can behave differently from a simple ASCII-only assumption in a locale with additional collation rules. When portability across systems matters, POSIX classes such as [[:lower:]] communicate intent and respect the active locale. If a script requires byte-oriented, locale-independent behavior, set its locale deliberately and document that choice rather than relying on the operator’s interactive environment.
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Binary input is not ordinary text
GNU grep can classify input containing null bytes or encoding errors as binary. In that situation it may suppress normal matching details and report that a binary file matches. If the data is known to be text but contains unusual bytes, GNU grep provides text-handling controls; use them only when you understand the encoding and the risk of terminal output containing control characters. The -I option is a safer choice for recursive source searches when binary files should simply be skipped.
When to use -P and when not to
grep -P enables Perl-compatible regular expressions in GNU grep. It can be useful for a feature unavailable in POSIX BRE or ERE, but support and diagnostics vary by build, distribution, and version. A script intended for many Unix-like systems should not assume that -P exists or behaves identically everywhere.
Before choosing PCRE, check whether a BRE or ERE expresses the requirement. ERE already handles ordinary grouping, alternation, and interval repetition, and is generally the better portability choice. Use PCRE only for a concrete PCRE feature and state that requirement in the script or documentation.
A practical decision framework
| Your requirement | Recommended form | Example |
|---|---|---|
| Find literal text exactly as typed | Fixed strings | grep -F 'a.b' file |
| Find a simple portable pattern | BRE (default) | grep '^timeout' file |
| Use readable alternatives or groups | ERE | grep -E '^(ERROR|WARN):' file |
| Require a whole word | -w with the appropriate matcher |
grep -iw 'warning' file |
| Require a complete line | -x, or both anchors |
grep -x 'READY' file |
| Search several patterns maintained separately | Repeated -e or -f |
grep -f patterns.txt file |
| Search a directory tree | Recursive grep with file filters | grep -RIn --include='*.conf' 'timeout' dir |
| Need a Perl-compatible-only feature | PCRE, after checking availability | grep -P 'pattern' file |
Troubleshooting patterns that do not behave as expected
The shell changed my pattern
Quote the pattern with single quotes. An unquoted * can be expanded to filenames, $ can trigger variable expansion, and a backslash can be consumed before grep receives it. If a pattern must contain a shell variable, isolate the variable deliberately and inspect the resulting value.
Parentheses or | do nothing
You may be using BRE syntax while writing an ERE. Add -E and write (...) and | plainly, or escape them for BRE as shown earlier. Do not add backslashes at random; each dialect has different operator rules.
The search matches too much
Add the missing boundary: ^ for the start, $ for the end, -w for a whole word, or -x for a complete line. Replace a broad dot with a bracket expression or a literal escaped dot when any-character matching is not intended.
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The search matches nothing
- Check case with
-i. - Check whether the text includes carriage returns, tabs, or trailing spaces.
- Confirm that you selected the intended dialect, especially after adding
-Eor-P. - For a pattern beginning with a hyphen, use
-e. - Test the expression against a small sample file before applying it recursively.
grep reports a binary file
The input contains bytes grep treats as binary. Skip such files with -I during recursive searches, or use an explicit text mode only when the file’s encoding and contents are known to be safe for terminal output.
A recursive search is slow or noisy
Narrow the tree and filenames with --include, exclude generated or vendor directories, skip binary files with -I, and use -l when you only need the names of matching files. A precise input set usually matters more than making the regex more complicated.
Quick Recap
A repeatable workflow
- Decide whether the search text is literal. If so, start with
-F. - Write the smallest pattern that states the requirement, then single-quote it.
- Use BRE for simple portable expressions or add
-Efor clear groups and alternatives. - Add boundaries such as
^,$,-w, or-xonly where the data model requires them. - Test against a short, representative file and inspect both matching and nonmatching examples.
- For many patterns, use repeated
-eoptions or a reviewed pattern file with-f. - For directory trees, constrain filenames and directories, decide how symbolic links should be handled, and account for binary data.
- In scripts, check grep’s exit status so “no match” is not confused with an input or option error.
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