Regex for grep (Perl with Posix support)

INFO

grep regex is Perl-compatible (PCRE): refer to the VSCode regex for the core syntax. Always use the -P flag to enable PCRE mode. Without it, grep falls back to BRE (like SED).

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Flags Cheatsheet

Flag	Description
-P	Enable PCRE mode — use this always for VSCode-like syntax
-E	Enable ERE mode (no PCRE, simpler alternative to -P)
-i	Case-insensitive match
-n	Show line numbers alongside matches
-r	Recurse into subdirectories
-l	Print only filenames containing a match (not the lines)
-v	Invert match — print lines that do not match
-o	Print only the matched portion, not the whole line
-c	Count matching lines

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How to Use Grep

Command	Description
grep -P 'pattern' file.txt	Match pattern in a file
grep -rP 'pattern' ./dir	Recursive search in a directory
grep -rPl 'pattern' ./dir	List only files with a match
grep -Pn 'pattern' file.txt	Show matches with line numbers
grep -Pv 'pattern' file.txt	Show lines that do NOT match
grep -Po 'pattern' file.txt	Extract only the matched text

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Grep-Specific Notes

• No \< / \> boundary syntax from SED — use \b as in VSCode
• Lookaheads and lookbehinds are supported with -P:

grep -P 'foo(?=bar)' file.txt     # "foo" only if followed by "bar"
grep -P 'foo(?!bar)' file.txt     # "foo" only if NOT followed by "bar"
grep -P '(?<=@)\w+' file.txt      # word after "@" (e.g. domain in an email)

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Exercises

Find lines containing an IPv4 address

grep -P '\b\d{1,3}(\.\d{1,3}){3}\b' file.txt

Extract all email addresses from a file

grep -Po '[\w.+-]+@[\w-]+\.[a-z]{2,}' file.txt

Find lines that do NOT contain a number

grep -Pv '\d' file.txt