Search Text with `grep` and Regular Expressions on Linux

grep can search for literal text or regular-expression patterns. With extended regular expressions, it becomes a compact tool for locating structured values, alternatives, prefixes, suffixes, and other text patterns.

Extended Regular Expressions

Use -E:

grep -E 'regex_pattern' file

Single quotes are often convenient in the shell because they prevent accidental expansion of regex metacharacters by the shell itself.

Match a Date-Like Pattern

grep -E '[0-9]{4}-[0-9]{2}-[0-9]{2}' log.txt

This matches text shaped like YYYY-MM-DD; it does not validate whether the date is a real calendar date.

Match Alternatives

grep -E 'error|warning' /var/log/syslog

For case-insensitive matching:

grep -Ei 'error|warning' application.log

Match the Start or End of a Line

grep -E '^ERROR' log.txt
grep -E 'success$' log.txt

^ anchors the pattern to the beginning of a line and $ to the end.

Character Classes and Repetition

grep -E 'file_[[:alnum:]_-]+\.txt' list.txt

POSIX character classes such as [[:digit:]], [[:alpha:]], and [[:alnum:]] can be more portable and expressive than ASCII-only ranges in some contexts.

Show Line Numbers and Search Recursively

grep -nE 'pattern' file.txt
grep -rEn 'pattern' /path/to/directory

Use -I when recursively searching source trees and you want to skip binary files:

grep -rInE 'TODO|FIXME' src/

Literal Search Instead of Regex

If the input should be treated as fixed text, use -F:

grep -F 'price[0]' file.txt

This prevents regex metacharacters from changing the meaning of the search string.

Conclusion

Use grep -E for regular expressions and grep -F for literal strings. Combine options such as -i, -n, and -r to control case sensitivity, line numbers, and recursive traversal.