Match "Mr." or "Ms." if word "her" is later in string
M(?(?=.*?\bher\b)s|r)\.
requires lookaround for IF condition
Regex examples
If-then-else
Lookaround
| Pattern | Meaning |
|---|---|
(?= ) |
Lookahead, if you can find ahead |
(?! ) |
Lookahead,if you can not find ahead |
(?<= ) |
Lookbehind, if you can find behind |
(?<! ) |
Lookbehind, if you can NOT find behind |
\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" |
\b\w+(?<!ing)\b |
Match words NOT ending in "ing" |
Atomic groups
| Pattern | Meaning |
|---|---|
(?>red|green|blue) |
Faster than non-capturing |
(?>id|identity)\b |
Match 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.
Non-capturing group
| Pattern | Meaning |
|---|---|
on(?:click|load) |
Faster than: on(click|load) |
| Use non-capturing or atomic groups when possible |
Back references
| Pattern | Matches |
|---|---|
(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 |
Groups
| Pattern | Meaning |
|---|---|
(in|out)put |
Match input or output |
\d{5}(-\d{4})? |
US zip code ("+ 4" optional) |
| Parser tries EACH alternative if match fails after group. |
Can lead to catastrophic backtracking.
Modifiers
| Pattern | Meaning |
|---|---|
(?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 |
(?-x) |
free-spacing mode OFF |
/regex/ismx |
Modify mode for entire string |
Scope
| Pattern | Meaning |
|---|---|
\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 |
^\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 |
Greedy versus lazy
| Pattern | Meaning |
|---|---|
* + {n,}greedy |
Match as much as possible |
<.+> |
Finds 1 big match in bold |
*? +? {n,}?lazy |
Match as little as possible |
<.+?> |
Finds 2 matches in bold |
Occurrences
| Pattern | Matches |
|---|---|
colou?r |
Match color or colour |
[BW]ill[ieamy's]* |
Match Bill, Willy, William's etc. |
[a-zA-Z]+ |
Match 1 or more letters |
\d{3}-\d{2}-\d{4} |
Match a SSN |
[a-z]\w{1,7} |
Match a UW NetID |
Shorthand classes
| Pattern | Meaning |
|---|---|
\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 |
Alternatives
| Pattern | Matches |
|---|---|
cat|dog |
Match cat or dog |
id|identity |
Match id or identity |
identity|id |
Match id or identity |
| Order longer to shorter when alternatives overlap |
Characters
| Pattern | Matches |
|---|---|
ring |
Match ring springboard etc. |
. |
Match a, 9, + etc. |
h.o |
Match hoo, h2o, h/o etc. |
ring\? |
Match ring? |
\(quiet\) |
Match (quiet) |
c:\\windows |
Match c:\windows |
Use \ to search for these special characters: |
|
[ \ ^ $ . | ? * + ( ) { } |
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