GREP cheat sheet

characters — what to seek
ring   matches ring, springboard, ringtone, etc.
.   matches almost any character

h.o   matches hoo, h2o, h/o, etc.

Use \ to search for these special characters:

[ \ ^ $ . | ? * + ( ) { }

ring\?   matches ring?

\(quiet\)   matches (quiet)

c:\\windows   matches c:\windows

alternatives — |   (OR)
cat|dog   match cat or dog
order matters if short alternative is part of longer
id|identity   matches id or identity

regex engine is "eager", stops comparing
as soon as 1st alternative matches

identity|id   matches id or identity
order longer to shorter when alternatives overlap
(To match whole words, see scope and groups.)
character classes — [allowed] or [^NOT allowed]
[aeiou]   match any vowel
[^aeiou]   match a NON vowel
r[iau]ng   match ring, wrangle, sprung, etc.
gr[ae]y   match gray or grey
[a-zA-Z0-9]   match any letter or digit
(In [ ] always escape . \ ] and sometimes ^ - .)
shorthand classes
\w   "word" character (letter, digit, or underscore)
\d   digit
\s   whitespace (space, tab, vtab, newline)
\W, \D, or \S, (NOT word, digit, or whitespace)

[\D\S] means not digit OR whitespace, both match

[^\d\s]   disallow digit AND whitespace

occurrences — ?  *  +  {n}  {n,}  {n,n}
?   0 or 1

colou?r   match color or colour

*   0 or more

[BW]ill[ieamy's]*   match Bill, Willy, William's etc.

+   1 or more

[a-zA-Z]+   match 1 or more letters

{n}   require n occurrences

\d{3}-\d{2}-\d{4}   match a SSN

{n,}   require n or more

[a-zA-Z]{2,}   2 or more letters

{n,m}   require n - m

[a-z]\w{1,7}   match a UW NetID

* greedy versus *? lazy
*  + and {n,} are greedy — match as much as possible
<.+>   finds 1 big match in <b>bold</b>
*?  +? and {n,}? are lazy — match as little as possible
<.+?>   finds 2 matches in <b>bold</b>
comments — (?#comment)
(?#year)(19|20)\d\d   embedded comment
(?x)(19|20)\d\d #year   free spacing & EOL comment

(see modifiers)

scope — \b  \B  ^  $
\b   "word" edge (next to non "word" character)

\bring   word starts with "ring", ex ringtone

ring\b   word ends with "ring", ex spring

\b9\b   match single digit 9, not 19, 91, 99, etc..

\b[a-zA-Z]{6}\b   match 6-letter words

\B   NOT word edge

\Bring\B   match springs and wringer

^ start of string   $ end of string

^\d*$   entire string must be digits

^[a-zA-Z]{4,20}$   string must have 4-20 letters

^[A-Z]   string must begin with capital letter

[\.!?"')]$   string must end with terminal puncutation

groups — ( )
(in|out)put   match input or output
\d{5}(-\d{4})?   US zip code ("+ 4" optional)
Locate all PHP input variables:


NB: parser tries EACH alternative if match fails after group.
Can lead to catastrophic backtracking.
back references — \n
each ( ) creates a numbered "back reference"
(to) (be) or not \1 \2   match to be or not to be
([^\s])\1{2}   match non-space, then same twice more   aaa, ...
\b(\w+)\s+\1\b   match doubled words
non-capturing group — (?: )   prevent back reference
on(?:click|load) is faster than on(click|load)
use non-capturing or atomic groups when possible
atomic groups — (?>a|b)   (no capture, no backtrack)
faster than non-capturing
alternatives parsed left to right without return
(?>id|identity)\b   matches id, but not identity

"id" matches, but "\b" fails after atomic group,
parser doesn't backtrack into group to retry 'identity'

If alternatives overlap, order longer to shorter.
lookahead — (?= ) (?! )   lookbehind — (?<= ) (?<! )
\b\w+?(?=ing\b)   match warbling, string, fishing, ...
\b(?!\w+ing\b)\w+\b   words NOT ending in "ing"
(?<=\bpre).*?\b   match pretend, present, prefix, ...
\b\w{3}(?<!pre)\w*?\b   words NOT starting with "pre"

(lookbehind needs 3 chars, \w{3}, to compare w/"pre")

\b\w+(?<!ing)\b   match words NOT ending in "ing"
(see LOOKAROUND notes below)
if-then-else — (?ifthen|else)
match "Mr." or "Ms." if word "her" is later in string
M(?(?=.*?\bher\b)s|r)\.   lookahead for word "her"
(requires lookaround for IF condition)
modifiers — i s m x
ignore case, single-line, multi-line, free spacing
(?i)[a-z]*(?-i)   ignore case ON / OFF
(?s).*(?-s)   match multiple lines (causes . to match newline)
(?m)^.*;$(?-m)   ^ & $ match lines not whole string
(?x)     #free-spacing mode, this EOL comment ignored
    \d{3}     #3 digits (new line but same pattern)
    -\d{4}     #literal hyphen, then 4 digits
(?-x) (?#free-spacing mode OFF)
/regex/ismx   modify mode for entire string

A few examples:

capture P content using if-then-else to allow for attributes in opening tag
(?s)<p(?(?=\s)\ .*?)>(.*?)</p>

The lookahead prevents matches on PRE, PARAM, and PROGRESS tags by only allowing more characters in the opening tag if P is followed by whitespace. Otherwise, ">" must follow "<p".


convert Firstname Lastname to Lastname, Firstname (& visa versa)
Pattern below uses lookahead to capture everything up to a space, characters, and a newline.
The 2nd capture group collects the characters between the space and the newline.
This allows for any number of names/initials prior to lastname, provided lastname is at the end of the line.

Find: (.*)(?= .*\n) (.*)\n

Repl: \2, \1\n — insert 2nd capture (lastname) in front of first capture (all preceding names/initials)

Reverse the conversion.

Find: (.*?), (.*?)\n — group 1 gets everything up to ", " — group 2 gets everything after ", "

Repl: \2 \1\n

lookaround groups are non-capturing
If you need to capture the characters that match the lookaround condition, you can insert a capture group inside the lookaround.

(?=(sometext))   the inner () captures the lookahead

This would NOT work: ((?=sometext))   Because lookaround groups are zero-width, the outer () capture nothing.

lookaround groups are zero-width
They establish a condition for a match, but are not part of it.
Compare these patterns:   re?d vs r(?=e)d
re?d — match an "r", an optional "e", then "d" — matches red or rd
r(?=e)d — match "r" (IF FOLLOWED BY "e") then see if "d" comes after "r"
fixed-width lookbehind
Most regex engines depend on knowing the width of lookbehind patterns. Ex: (?<=h1) or (?<=\w{4}) look behind for "h1" or for 4 "word" characters.
This limits lookbehind patterns when matching HTML tags, since the width of tag names and their potential attributes can't be known in advance.
variable-width lookbehind
.NET and JGSoft support variable-width lookbehind patterns. Ex: (?<=\w+) look behind for 1 or more word characters.
The first few examples below rely on this ability.
match text bound by simple HTML tags   (NB: <\w+> does not match tags with attributes.)

Lookaround groups define the context for a match. Here, we're seeking   .*   ie., 0 or more characters.
A positive lookbehind group   (?<= . . . )   preceeds. A positive lookahead group   (?= . . . )   follows.
These set the boundaries of the match this way:

In other words, advance along string until an opening HTML tag preceeds. Match chars until its closing HTML tag follows.
The tags themselves are not matched, only the text between them.

To span multiple lines, use the (?s) modifier.   (?s)(?<=<cite>).*(?=</cite>)   Match <cite> tag contents, regardless of line breaks.

match text bound by HTML tags, including tags with attributes (not nested, though)
(?<=<(\w+?)\ ?.*?>).*(?=</\1>)

As in example above, the first group (\w+) captures the presumed tag name, then an optional space and other characters \ ?.*? allow for attributes before the closing >.

match text bound by HTML tags when a class attribute = "red"
(?<=<(\w+?)\ ?.*?class=".*?\bred\b.*?".*?>).*(?=</\1>)
match complex opening & closing xhtml tags and all text between

Here, the first group captures only the tag name. The tag's potential attributes are outside the group.

IF condition — phone number w/optional parentheses around area code (and optional space after closing parens)
(\()?\d{3}(?(1)\) ?|[- \.])\d{3}[- \.]\d{4}

The IF condition can be set by a backreference (as here) or by a lookaround group.

(\()?\d{3}   optional group ( )? matches "(" prior to 3-digit area code \d{3} — group creates back reference #1
(?(1)\) ?|[-/ \.])   (1) refers to group 1, so if "(" exists, match ")" followed by optional space, else match one of these: "- / . "
\d{3}[- \.]\d{4}   rest of phone number
groups can be named (assume a file of lastname, firstname altered using "preg_replace()")
(?#find)(\b.+), (\b.*\b)   (?#replace)\2 \1 
(?#find)(?P<lname>\b.+), (?P<fname>\b.*\b)   (?#replace) (?P=fname) (?P=lname)

For a quick overview:

For a good tutorial: