Do Boolean operators have to be in capital letters?

In PubMed, yes: its user guide says to enter Boolean operators in uppercase characters. Web of Science's help says the opposite, that its search operators are not case sensitive. Writing operators in capitals satisfies both, and it makes strings easier to read, so it is the safe habit.

Does Google Scholar support Boolean searching?

Only in a basic form. Its help page documents an author: operator and says the advanced search window lets you search the author, title and publication fields and limit results by date. Google's help page documents no truncation, wildcard or proximity operators. Haddaway and colleagues (2015) report that search strings are limited to 256 characters, that only the first 1,000 results are displayed and that results cannot be exported in bulk. It is useful for finding known items and for citation chasing, not as the main database of a reproducible search.

How many results should a good search string return?

There is no target number. A systematic review search favours sensitivity, so it returns many more records than will be included. For a thesis chapter, aim for a set you can realistically scan. Judge the string by whether it retrieves the key papers you already know and whether the first few pages are mostly on topic.

When should I use NOT?

Rarely. NOT removes every record that mentions the excluded word, including relevant ones that mention it in passing. A search for adolescents NOT adults drops every study that compares the two groups. It is safer to narrow with an additional AND concept or a field restriction, and to exclude unwanted records at screening.

What is the difference between keywords and subject headings?

Keywords match the words authors used in the title and abstract. Subject headings, such as MeSH in PubMed or the ERIC Thesaurus descriptors, are standardized labels added by indexers, so they find papers on a concept whatever wording the authors chose. A thorough search combines both with OR for each concept, because recent records may not be indexed yet and not every concept has a heading.

Boolean Search Strings for Academic Databases: Syntax, Examples and Translation

A search box that accepts natural language is convenient, but it gives you no control over what is matched and no way to report what you did. A Boolean string gives you both. It is also the one part of a literature search that a supervisor or reviewer can read and evaluate directly.

This guide builds one string step by step and then translates it for the main academic platforms, since the same string pasted into a different database can run without an error message and still return the wrong set.

The building blocks

Element	What it does	Example
OR	Either term. Broadens. Use between synonyms	`teenager OR adolescent`
AND	Both terms. Narrows. Use between concepts	`adolescent AND anxiety`
NOT (AND NOT in Scopus)	Excludes. Use sparingly	`mindfulness NOT yoga`
Parentheses	Control the order of processing	`(teenager OR adolescent) AND anxiety`
Quotation marks	Exact phrase	`"social anxiety"`
Asterisk	Truncation: any ending	`adolescen*` finds adolescent, adolescents, adolescence
Wildcard	One character inside a word	`wom?n`, `organi?ation` (symbol varies by database)
Proximity operator	Terms within n words of each other	`mindfulness NEAR/3 program*` (syntax varies)
Field code	Restrict to title, abstract, subject heading and so on	`anxiety[tiab]`, `TITLE-ABS-KEY(anxiety)`

Why parentheses are not optional

Databases do not agree on the order in which operators are processed.

Platform	Documented order of processing
PubMed	Left to right
Scopus	OR, then the proximity operators `W/n` and `PRE/n`, then AND, then AND NOT
Web of Science	NEAR/x, then SAME, then NOT, then AND, then OR
ProQuest	PRE, NEAR, then AND, OR, NOT

Scopus is in the middle of changing this. A Scopus blog post of 24 March 2025 announced a new precedence of AND NOT, then AND, then OR, "starting in late 2025" and to be finalized "in early 2026". The Scopus advanced-search help page, last updated 24 August 2026, still documents the OR-first order shown above, so the same unbracketed string may behave differently depending on when and where it is run. Check the help page before relying on precedence.

Take the unbracketed string mindfulness AND teenager OR adolescent. PubMed reads it left to right, combining mindfulness with teenager and then adding every record that mentions adolescent. Web of Science and ProQuest process AND before OR and give the same oversized result; ProQuest's own help illustrates this with dog OR cat AND food OR nutrition, which it processes as dog OR (cat AND food) OR nutrition. Scopus under the OR-first rule returns what you meant, and under the announced rule it would behave like Web of Science. With parentheses, mindfulness AND (teenager OR adolescent) returns the intended set everywhere. Put every OR group in parentheses and you never have to remember the table.

EBSCOhost is left out of this table deliberately. EBSCO documents its operators in EBSCO Connect, whose pages did not render to us on the day of checking (21 September 2026) and are not captured in the Internet Archive, so we could not confirm its precedence or its wildcard symbols first-hand. Look them up in EBSCO Connect, or ask your librarian, for the database you are actually searching.

Build a string in four steps

The example question: do school-based mindfulness programmes reduce anxiety in adolescents?

Step 1: Identify the concepts

Use the main nouns of the question. Leave out words such as "effect", "reduce" or "impact", which authors express in too many ways. Here there are four concepts: mindfulness, adolescents, anxiety, school setting. Start with the three or four that matter most. Each AND concept you add removes records, including relevant ones.

Step 2: List synonyms, spellings and related terms

Concept 1	Concept 2	Concept 3	Concept 4
mindfulness	adolescent, adolescents, adolescence	anxiety	school, schools
mindful	teenager, teen, teens	anxious	classroom
MBSR	youth	worry	school-based
meditation	young people	internalizing, internalising	curriculum
	secondary school students, high school students

Sources for synonyms: the keywords and abstracts of three or four papers you already have, the database thesaurus (MeSH, the ERIC Thesaurus, APA Thesaurus of Psychological Index Terms), and British and American spellings.

Step 3: Combine

OR within a column, AND between columns, with truncation and phrases:

(mindful* OR MBSR OR meditat*)
AND (adolescen* OR teen* OR youth OR "young people" OR "high school student*" OR "secondary school student*")
AND (anxi* OR worry OR internali?ing)
AND (school* OR classroom* OR curricul*)

Step 4: Test and adjust

Known-item test. Pick five relevant papers you already have. Does the string retrieve each one? If one is missing, look at its title, abstract and subject headings to see which concept block failed, and add the missing term.
Too many irrelevant results? Restrict keyword blocks to title and abstract, replace AND with a proximity operator between two concepts, or drop the loosest synonym (here, probably youth or meditat*). Check the platform first: Web of Science does not accept AND in a query that also uses NEAR, so there the proximity pairing has to be run as its own search line.
Too few? Remove the least essential concept block. In this example the school block is the first candidate, because many abstracts describe the setting without those words.
Check truncation. teen* is safe. A short stem such as cat* would match catalyst and category. PubMed's user guide requires at least four characters before the first wildcard. Other platforms set their own minimum, so check the help page for the one you are using.

Librarians reviewing search strategies for systematic reviews use the PRESS checklist (McGowan et al., 2016), which looks at the same things. Its six retained elements are translation of the research question; Boolean and proximity operators; subject headings; text word search; spelling, syntax and line numbers; and limits and filters.

Translate the string for each database

The logic stays the same. Field codes, proximity operators and wildcards change.

PubMed

(mindfulness[mh] OR mindful*[tiab] OR MBSR[tiab] OR meditat*[tiab])
AND (adolescent[mh] OR adolescen*[tiab] OR teen*[tiab] OR youth[tiab] OR "young people"[tiab])
AND (anxiety[mh] OR anxi*[tiab] OR worry[tiab])
AND (schools[mh] OR school*[tiab] OR classroom*[tiab] OR curricul*[tiab])

[mh] searches MeSH headings and automatically includes narrower headings. [tiab] searches title and abstract.
Operators must be uppercase. Using quotation marks, truncation or a field tag switches off PubMed's automatic term mapping for that term, which is why MeSH terms are added explicitly.
Proximity searching was added in late 2022 with the form "mindfulness program"[tiab:~3], meaning the quoted words appear in any order with at most three words between them. It works only in the title, title/abstract and affiliation fields. If the quoted terms include an asterisk, PubMed ignores the proximity operator.
The asterisk stands for zero or more characters and can be used inside a word (organi*ation). There is no single-character wildcard.

Scopus

TITLE-ABS-KEY(
  (mindful* OR mbsr OR meditat*)
  AND (adolescen* OR teen* OR youth OR "young people")
  AND (anxi* OR worry OR internalizing OR internalising)
  AND (school* OR classroom* OR curricul*)
)

TITLE-ABS-KEY() searches abstracts, keywords and document titles. TITLE-ABS() searches abstracts and titles only.
Proximity: W/n for within n words in any order, PRE/n for the first term preceding the second within n words. Example: mindful* W/3 program*.
Straight double quotation marks give a loose phrase: punctuation is ignored except hyphens and dots, plurals and spelling variants are included, and wildcards still work inside the quotes. Curly braces give an exact phrase, including stop words, spaces and punctuation: {oyster toadfish}. Scopus warns that curly or smart quotation marks pasted from a word processor can cause a query parsing error, so check that your quotes are straight ASCII quotes.
Exclusion is written AND NOT.
There is no subject-heading field like PubMed's [mh] in this string, so the synonym lists carry the whole search and need to be fuller.

Web of Science

TS=((mindful* OR MBSR OR meditat*)
AND (adolescen* OR teen* OR youth OR "young people")
AND (anxi* OR worry OR internali?ing)
AND (school* OR classroom* OR curricul*))

TS= is the topic field. In the Core Collection, Topic searches the title, abstract, author keywords and Keywords Plus fields.
Proximity: NEAR/n, any order. Example: mindful* NEAR/3 program*. NEAR without /n finds terms within 15 words of each other.
AND and NEAR cannot be mixed. Web of Science's help states that "You cannot use the AND operator in queries that include the NEAR operator", and gives TS = (Germany NEAR/10 (monetary AND union)) as an invalid query. If you want proximity between two concepts here, write each pairing on its own line and combine the result sets.
Wildcards: * for any group of characters including none, ? for any single character, $ for zero or one character. Web of Science's own example is flavo$r, which finds flavor and flavour.
Note which collection and which editions you searched, because Web of Science subscriptions differ between institutions.

EBSCOhost (ERIC, APA PsycInfo, CINAHL, Academic Search)

(mindful* OR MBSR OR meditat*)
AND (adolescen* OR teen* OR youth OR "young people")
AND (anxi* OR worry OR internalizing OR internalising)
AND (school* OR classroom* OR curricul*)

The concept structure carries across unchanged. Enter each concept block on its own row in Advanced Search and set the field for that row, which avoids typing field codes altogether.
Each EBSCO database has its own thesaurus. Add subject terms from the one you are searching, combined with OR alongside the keywords for that concept.
Look up the symbols before you rely on them. EBSCO's field codes, proximity operators, wildcard characters and truncation minimum are documented in EBSCO Connect, and they are not identical to Scopus's or Web of Science's. We were unable to load those pages on 21 September 2026 and so do not reproduce the symbols here rather than risk getting them wrong. Note also that the variant spellings are spelled out above instead of using a wildcard, which works on any platform.

ProQuest (Dissertations & Theses Global, Education Database and others)

noft((mindful* OR MBSR OR meditat*)
AND (adolescen* OR teen* OR youth OR "young people")
AND (anxi* OR worry OR internali?ing)
AND (school* OR classroom* OR curricul*))

noft() is "Anywhere except full text": it searches the full bibliographic record but not the full text. ProQuest's platform default is "Anywhere", which does include the full text, and your library's administrator may have changed the default to noft. Full-text searching returns much larger, noisier sets, so set this deliberately rather than inheriting it.
Proximity: NEAR/n or N/n in any order, PRE/n or P/n in the order typed. Without a number, NEAR defaults to a maximum of four intervening words.
* truncates, and by default retrieves at most five extra characters, so anxi* finds anxiety and anxious but not anxiousness. Write anxi[*10] to allow up to ten (the maximum is 20).
? replaces any single character inside or at the right end of a word, and ProQuest notes that one ? matches zero or one character, so cat? returns both cat and cats.

Google Scholar

mindfulness adolescents OR teenagers anxiety school OR classroom

A space means AND. Google's help page documents no truncation or wildcard operator, so type the variants you need. Haddaway and colleagues (2015) report a 256-character limit on search strings and that only the first 1,000 results are displayed.
Google's help says the advanced search window "lets you search in the author, title, and publication fields, as well as limit your search results by date". Restricting to the title field cuts noise considerably. The one documented operator is author:, as in author:"d knuth".
The same paper notes that results are displayed "with no details of the means by which they are ordered" and cannot be exported in bulk, so Google Scholar is best treated as a supplementary source and a tool for forward citation searching ("Cited by").

Syntax summary

Platform	Title/abstract field	Proximity, any order	Proximity, in order	One character	Zero or one character
PubMed	`[tiab]`	`"a b"[tiab:~n]`	none documented	none documented	none documented
Scopus	`TITLE-ABS()` or `TITLE-ABS-KEY()`	`W/n`	`PRE/n`	—	—
Web of Science	`TS=` (topic)	`NEAR/n`	none documented	`?`	`$`
ProQuest	`noft()`	`NEAR/n`, `N/n`	`PRE/n`, `P/n`	—	`?`
Ovid (MEDLINE, Embase, PsycInfo)	`.ti,ab.`	`adjn`	`adj` (adjacent words only)	`#`	`?`

A dash means the platform's help page does not define that symbol in the pages cited below, not that no such symbol exists. EBSCOhost is omitted for the reason given in its section above. Database interfaces change, and Scopus is mid-change: check the platform's own help page before finalizing a search for publication.

Record what you ran

For every database, save:

the platform and database name (for example "APA PsycInfo via EBSCOhost"),
the date,
the complete string exactly as run, including limits,
the number of results.

PRISMA 2020 asks for the full strategy for every database, and these counts feed the first box of the PRISMA flow diagram. For a thesis chapter, the same record lets you write two accurate sentences about your search and rerun it before submission to catch new publications. Most databases let you save searches and set alerts in a free personal account.

Common mistakes

Pasting one string into every database. Field tags from PubMed are ignored or misread elsewhere, and Scopus's W/3 means nothing in Web of Science, which wants NEAR/3.
ANDing synonyms. teenager AND adolescent finds only papers that use both words.
Forgetting spelling variants such as behaviour and behavior, or programme and program.
Truncating too early. soc* matches social, society, soccer and Socrates.
Searching full text by default in ProQuest or a publisher platform, then concluding that the topic is enormous.
Using NOT to remove a population, which also removes comparative studies.
Relying on keywords only where the database has a thesaurus.
Not testing against known papers. A string that misses the three articles your supervisor gave you is not finished.

Wonders suggests keywords for your topic and builds the Boolean query with you, across 320M+ scholarly works. It is a good way to draft the concept blocks and see quickly whether a synonym adds relevant papers. For databases your review must report, translate the string using the syntax table above.

References

All pages below were checked on 21 September 2026. The two Web of Science pages return 403 to non-browser requests, so they were read through the Internet Archive; the snapshot dates and the pages' own "updated" dates are given.

Clarivate. Web of Science help: Search Operators (page updated 17 October 2025; read via the Internet Archive snapshot of 6 April 2026). https://webofscience.zendesk.com/hc/en-us/articles/20016122409105-Search-Operators
Clarivate. Web of Science help: Search Rules, including wildcards (page updated 23 July 2026; read via the Internet Archive snapshot of 21 September 2026). https://webofscience.zendesk.com/hc/en-us/articles/25350084904721-Search-Rules
Clarivate. Web of Science Core Collection Search Fields (page updated 6 August 2025; read via the Internet Archive). https://webofscience.zendesk.com/hc/en-us/articles/26916258216209-Web-of-Science-Core-Collection-Search-Fields
EBSCO Connect. Searching with Boolean operators. https://connect.ebsco.com/s/article/Searching-with-Boolean-Operators
EBSCO Connect. Searching with wildcards in EDS and EBSCOhost. https://connect.ebsco.com/s/article/Searching-with-Wildcards-in-EDS-and-EBSCOhost
Elsevier. How can I best use the Advanced search? Scopus Support Center (last updated 24 August 2026). https://www.elsevier.support/scopus/answer/how-can-i-best-use-the-advanced-search
Elsevier Scopus Blog. (2025, March 24). Boolean searches in Scopus: understanding operator precedence and best practices. https://blog.scopus.com/boolean-searches-in-scopus-understanding-operator-precedence-best-practices/
Google Scholar. Search help. https://scholar.google.com/intl/en/scholar/help.html
Haddaway, N. R., Collins, A. M., Coughlin, D., & Kirk, S. (2015). The role of Google Scholar in evidence reviews and its applicability to grey literature searching. PLOS ONE, 10(9), e0138237. https://doi.org/10.1371/journal.pone.0138237
Lefebvre, C., Glanville, J., Briscoe, S., Featherstone, R., Littlewood, A., Metzendorf, M.-I., Noel-Storr, A., Paynter, R., Rader, T., Thomas, J., & Wieland, L. S. Chapter 4: Searching for and selecting studies (last updated March 2025). In Cochrane Handbook for Systematic Reviews of Interventions, version 6.5.1. https://www.cochrane.org/authors/handbooks-and-manuals/handbook/current/chapter-04
McGowan, J., Sampson, M., Salzwedel, D. M., Cogo, E., Foerster, V., & Lefebvre, C. (2016). PRESS Peer Review of Electronic Search Strategies: 2015 guideline statement. Journal of Clinical Epidemiology, 75, 40–46. https://doi.org/10.1016/j.jclinepi.2016.01.021
National Library of Medicine. PubMed User Guide. https://pubmed.ncbi.nlm.nih.gov/help/
National Library of Medicine. (2022). PubMed update: Proximity search now available in PubMed. NLM Technical Bulletin, Nov–Dec 2022(449), e4. https://www.nlm.nih.gov/pubs/techbull/nd22/nd22_pubmed_proximity_search_available.html
ProQuest. ProQuest Platform: Search. https://proquest.libguides.com/proquestplatform/search
ProQuest. ProQuest Platform: Search Tips (last updated 31 August 2026). https://proquest.libguides.com/proquestplatform/tips
Wolters Kluwer. Ovid MEDLINE database guide: Advanced searching. https://ospguides.ovid.com/OSPguides/medline.htm