====== Regular Expressions ======

Same pattern means different things in different tools. This is the table
that resolves it.

===== Regex Flavors by Tool =====

^ Tool ^ Default ^ Upgrade flag ^
| ''grep'' | BRE | ''-E'' for ERE, ''-P'' for PCRE |
| ''sed'' | BRE | ''-E'' (or ''-r'') for ERE |
| ''awk'' | ERE-ish | gawk has more |
| ''rg'' | Rust regex | no backrefs or lookaround |
| ''perl'' | PCRE | |
| <nowiki>[[ =~ ]]</nowiki> in bash | ERE | |

===== The Differences That Matter =====

^ Feature ^ BRE (grep/sed default) ^ ERE (''-E'', awk, bash ''=~'') ^
| ''+'', ''?'' quantifiers | ''\+'', ''\?'' (escaped = special) | bare ''+'' ''?'' |
| groups | ''\( \)'' | ''( )'' |
| alternation | ''\\|'' | ''<nowiki>|</nowiki>'' |
| ''{n,m}'' counts | ''\{n,m\}'' | ''{n,m}'' |
| backrefs in pattern | ''\1'' | ''\1'' for ERE search; not in rg |

Escaped-vs-bare flips between BRE and ERE: in BRE, ''(a)'' is literal parens
and ''\(a\)'' is a group; in ERE it's the opposite. **When unsure, use ''-E''.**

===== Universal building blocks =====

<code regex>
.        any char            \.        literal dot
* + ?    0+, 1+, 0-1         ^ $       line start/end
[abc]    set                 [^abc]    negated set
[a-z]    range               {n,m}     counts: {3} {2,} {2,5}
x|y      alternation (ERE)   (x|y)     group
</code>

POSIX classes (work everywhere, even BRE):

<code regex>
[[:digit:]]  [[:alpha:]]  [[:alnum:]]  [[:space:]]  [[:upper:]]  [[:lower:]]
[[:punct:]]  [[:print:]]  [[:blank:]]  [[:xdigit:]]
</code>

GNU extras (work in GNU grep/sed both flavors): ''\b'' word boundary,
''\<''/''\>'' word edges, ''\w'' word char, ''\s'' whitespace, ''\B''.

PCRE/Rust extras (''grep -P'', ''rg'', perl): ''\d \D \s \S \w \W'',
non-greedy ''*?'', named groups ''(?P<n>..)'', lookahead ''(?=..)'' ''(?!..)''.
**Note:** ''rg'''s default engine supports NO lookaround/backrefs: add
''rg -P'' to get PCRE2 when you need them.

===== Practical translations =====

<code bash>
# same task, three tools
grep -E '^(Mon|Tue)' f         sed -nE '/^(Mon|Tue)/p' f    rg '^(Mon|Tue)' f

# BRE version of the ERE "(a|b)+"
grep 'x\(a\|b\)\+y'            # hard to read: use -E instead

# IPs: ERE
grep -oE '[0-9]{1,3}(\.[0-9]{1,3}){3}'
# BRE equivalent (don't do this)
grep -o '[0-9]\{1,3\}\(\.[0-9]\{1,3\}\)\{3\}'
</code>

===== Substitution notes =====

  * sed backrefs in replacement: ''\1''-''\9'', ''&'' = whole match.
  * rg capture replace: ''rg 'a(\d+)' -r 'num$1''' uses ''$1''.
  * Case transforms (GNU sed): ''sed 's/\(.*\)/\U\1/''' uppercases.

===== When Regex Fails =====

  * Pattern has ''<nowiki>|</nowiki>''/''(''/''{'' and you did not mean regex: ''grep -F''.
  * Greedy ''.*'' matched too much: use ''[^x]*'' or PCRE ''.*?''.
  * Matching across lines: ''grep -z'', ''perl -0777'', or awk paragraph mode.
  Line-oriented tools will not do it with ''$''/''.'' alone.


===== See Also =====

  * [[terminal:scripting:start|Scripting Index]]
  * [[terminal:tools:start|CLI Tools]]
  * [[terminal:start|Return to terminal]]
  * [[start|Return to wiki home]]